diff --git a/ChangeLog b/ChangeLog
index 0a58eb002..59666096b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,7 +5,11 @@
 	method to only fire one timer (excluding the private housekeeping one).
 	Changing to only fire one timer actually allows the code to be simpler
 	so it's a good update in its own right, not just a OSX compatibility
-	tweak.
+	tweak.  I also made incrementing repeated timers a bit more robust,
+	checking that they provide a valid increment time interval and
+	removing them from the loop if they don't.
+	* Source/NSTimer.m: slightly tidy/clarify initialisation code and
+	documentation.
 
 2009-09-08 Richard Frith-Macdonald <rfm@gnu.org>
 
diff --git a/Source/NSRunLoop.m b/Source/NSRunLoop.m
index 64c10d0bb..2669bea3e 100644
--- a/Source/NSRunLoop.m
+++ b/Source/NSRunLoop.m
@@ -62,6 +62,7 @@
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
 #endif
+#include <math.h>
 #include <time.h>
 #include <limits.h>
 #include <string.h>		/* for memset() */
@@ -813,6 +814,20 @@ static inline BOOL timerInvalidated(NSTimer *t)
   GSIArray	timers;
   unsigned      i;
 
+  if ([timer isKindOfClass: [NSTimer class]] == NO
+    || [timer isProxy] == YES)
+    {
+      [NSException raise: NSInvalidArgumentException
+		  format: @"[%@-%@] not a valid timer",
+	NSStringFromClass([self class]), NSStringFromSelector(_cmd)];
+    }
+  if ([mode isKindOfClass: [NSString class]] == NO)
+    {
+      [NSException raise: NSInvalidArgumentException
+		  format: @"[%@-%@] not a valid mode",
+	NSStringFromClass([self class]), NSStringFromSelector(_cmd)];
+    }
+
   NSDebugMLLog(@"NSRunLoop", @"add timer for %f in %@",
     [[timer fireDate] timeIntervalSinceReferenceDate], mode);
 
@@ -875,10 +890,38 @@ updateTimer(NSTimer *t, NSDate *d, NSTimeInterval now)
       NSTimeInterval	ti = [d timeIntervalSinceReferenceDate];
       NSTimeInterval	increment = [t timeInterval];
 
-      ti += increment;
-      while (ti < now)
+      if (increment <= 0.0)
 	{
-	  ti += increment;
+	  /* Should never get here ... unless a subclass is returning
+	   * a bad interval ... we return NO so that the timer gets
+	   * removed from the loop.
+	   */
+	  NSLog(@"WARNING timer %@ had bad interval ... removed", t);
+	  return NO;
+	}
+
+      ti += increment;	// Hopefully a single increment will do.
+
+      if (ti < now)
+	{
+	  NSTimeInterval	add;
+
+	  /* Just incrementing the date was insufficieint to bring it to
+	   * the current time, so we must have missed one or more fire
+	   * opportunities, or the fire date has been set on the timer.
+	   * If a fire date long ago has been set and the increment value
+	   * is really small, we might need to increment very many times
+	   * to get the new fire date.  To avoid looping for ages, we
+	   * calculate the number of increments needed and do them in one
+	   * go.
+	   */
+	  add = floor((now - ti) / increment);
+	  ti += (increment * add);
+	  if (ti < now)
+	    {
+	      add++;
+	      ti += increment;
+	    }
 	}
       d = [[NSDate alloc] initWithTimeIntervalSinceReferenceDate: ti];
       [t setFireDate: d];
diff --git a/Source/NSTimer.m b/Source/NSTimer.m
index da03012b9..6165b65e9 100644
--- a/Source/NSTimer.m
+++ b/Source/NSTimer.m
@@ -69,22 +69,21 @@ static Class	NSDate_class;
   return nil;
 }
 
-/**
- * <init />
+/** <init />
  * Initialise the receive, a newly allocated NSTimer object.<br />
- * The fd argument specifies an initial fire date ... if it is not
- * supplied (a nil object) then the ti argument is used to create
- * a start date relative to the current time.<br />
  * The ti argument specifies the time (in seconds) between the firing.
  * If it is less than or equal to 0.0 then a small interval is chosen
  * automatically.<br />
+ * The fd argument specifies an initial fire date copied by the timer...
+ * if it is not supplied (a nil object) then the ti argument is used to
+ * create a start date relative to the current time.<br />
  * The f argument specifies whether the timer will fire repeatedly
  * or just once.<br />
  * If the selector argument is zero, then then object is an invocation
  * to be used when the timer fires.  otherwise, the object is sent the
  * message specified by the selector and with the timer as an argument.<br />
- * The fd, object and info arguments will be retained until the timer is
- * invalidated.<br />
+ * The object and info arguments will be retained until the timer is
+ * invalidated.
  */
 - (id) initWithFireDate: (NSDate*)fd
 	       interval: (NSTimeInterval)ti
@@ -93,26 +92,30 @@ static Class	NSDate_class;
 	       userInfo: (id)info
 		repeats: (BOOL)f
 {
-  if (ti <= 0)
+  if (ti <= 0.0)
     {
       ti = 0.0001;
     }
-  _interval = ti;
   if (fd == nil)
     {
       _date = [[NSDate_class allocWithZone: NSDefaultMallocZone()]
-        initWithTimeIntervalSinceNow: _interval];
+        initWithTimeIntervalSinceNow: ti];
     }
   else
     {
-      _date = [fd copy];
+      _date = [fd copyWithZone: NSDefaultMallocZone()];
     }
   _target = RETAIN(object);
   _selector = selector;
   _info = RETAIN(info);
-  _repeats = f;
-  if (_repeats == NO)
+  if (f == YES)
     {
+      _repeats = YES;
+      _interval = ti;
+    }
+  else
+    {
+      _repeats = NO;
       _interval = 0.0;
     }
   return self;
@@ -266,7 +269,6 @@ static Class	NSDate_class;
 - (void) invalidate
 {
   /* OPENSTEP allows this method to be called multiple times. */
-  //NSAssert(_invalidated == NO, NSInternalInconsistencyException);
   _invalidated = YES;
   if (_target != nil)
     {