Calling performSelector:onThread:withObject:waitUntilDone: more than once on same thread results in delay - objective-c

Question

For code like:
// Code in some object that will do work for an application:
- (BOOL)shouldBeRunning
{
[lockRunning lock];
BOOL shouldBeRunning= shouldRun;
[lockRunning unlock];
return shouldBeRunning;
}
- (void)stopRunning
{
[lockRunning lock];
shouldRun= FALSE;
[lockRunning unlock];
}
- (void)threadEntryPoint:(id)object
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
// From an example I saw awhile back:
// A runloop with no sources returns immediately from runMode:beforeDate:
// That will wake up the loop and chew CPU. Add a dummy source to prevent it.
NSMachPort *dummyPort = [[NSMachPort alloc] init];
[runLoop addPort:dummyPort forMode:NSDefaultRunLoopMode];
[dummyPort release];
[pool release];
while ([self shouldBeRunning])
{
NSAutoreleasePool *loopPool = [[NSAutoreleasePool alloc] init];
[runLoop runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
[loopPool drain];
}
}
- (BOOL)startRunning:(NSError **)errorPtr
{
[self stopRunning]; // Stop if we are already running.
[runWorker release];
runWorker= [[NSThread alloc] initWithTarget:self selector:#selector(threadEntryPoint:) object:nil];
if(!runWorker)
return (FALSE);
// Start up the thread.
shouldRun= TRUE;
[runWorker start];
return TRUE;
}
- (void)doLotsOfStuff
{
// Some operation that is long and intensive
// that should be done in the background.
// This function will call the app delegate, which will display the
// results. It will also notify the app on completion.
}
- (void)doStuff
{
// Commented out for illustrative purposes.
//[self startRunning]; // Fire thread up.
[self performSelector:#selector(doLotsOfStuff) onThread:runWorker withObject:nil waitUntilDone:NO];
}
// Out in the delegate:
- (void)applicationDidFinishLaunching:(NSNotification*)aNotification
{
// Do setup....
[workObject startRunning]; // Start the worker thread in the worker object.
}
- (void)buttonHandler:(id)sender
{
[workObject doStuff];
}
So, in the application there is a button. The user will press it, and a task will run on a worker thread. The task will provide feedback to the application. In this case, the button is disabled until the task completes. I just do not want to show all that code.
With the code as written, if I press the button once, the task runs without delay. Often, a second button press yields the same result. However, sometimes the second press, but almost always the third or after, will result in a significant delay in performing the task. I put debug statements in and can observe that the code does the performSelector on the thread, then there is a delay, and finally the task runs.
If I uncomment the line in doStuff that re-creates the thread (making the one in applicationDidFinishLaunching redundant), of course it works perfectly every time.
From what I can tell, the thread is getting into an unresponsive state.
Any ideas on what might be going on? Anything obviously wrong with the setup and handling code? Any input appreciated.