Yes, he says the issue is with Reaper.

I use Reaper also and will occasionally get stutter on playback if I have too many plugins active, it runs out of CPU.

Within reaper you should bring up the performance meter before pressing play. You do that with Ctrl->Alt->p

Play your track and watch the monitor. It will show the amount of CPU being used by each track. Once you see which track has the problem you can narrow down which plugin is causing it.