All the threads will perform endless wait by adding Synchronizing Timer.

This issue is much easier reproduced under a heavy load. I was using 100 threads to a simple HTTP request sampler with adding a Synchronizing Timer, Groupsize set to 100 as well.

please see the attachment which is the stack trace of 100 threads doing a endless wait.

Some initial invstigation for this issue:

Here is the current source code:

public long delay() {
        synchronized (sync) {
            timerCounter[0]++;
            final int groupSz = getGroupSize();
            final int count = timerCounter[0];
            if (
                (groupSz == 0 && count >= JMeterContextService.getNumberOfThreads())
                ||
                (groupSz > 0 && count >= groupSz)
                ) {
                sync.notifyAll();
            } else {
                try {
                    sync.wait();
                } catch (InterruptedException e) {
                    log.warn(e.getLocalizedMessage());
                }
            }
            timerCounter[0]=0; // Reset for next time
        }
        return 0;
    }

The main problem from my perspective is after one thread call sync.notifyAll(); ,which means after the lock has been released,it will execute timerCounter[0]=0;(correct me if i am wrong)

After chaning the code as following(move "timerCounter[0]=0;" in front of "sync.notifyAll();", it does not impact any business impact, but make more sense to me), and the issue disappear:
public long delay() {
        synchronized (sync) {
            timerCounter[0]++;
            final int groupSz = getGroupSize();
            final int count = timerCounter[0];
            if (
                (groupSz == 0 && count >= JMeterContextService.getNumberOfThreads())
                ||
                (groupSz > 0 && count >= groupSz)
                ) {
                timerCounter[0]=0; // Reset for next time
                sync.notifyAll();
            } else {
                try {
                    sync.wait();
                } catch (InterruptedException e) {
                    log.warn(e.getLocalizedMessage());
                }
            }
        }
        return 0;
    }

Please review the code and make according changes with this issue. BTW,It can be reproduced on 2.3.4 release as well.
Hope this helps!