Hi, this sounds identical to a problem I had, using Reaper on a MacBook Pro.
It first occurred a year or two ago, and led me to stop using Reaper, as nothing I tried would fix the issue, and the software was pretty much unusable. But a few months later, I tried again, and the problem seemed to have fixed itself.
Then just recently the problem recurred. Following a hint here -
http://forum.cockos.com/showthread.php?t=87630 - I went into Project Settings, checked the "Project sample rate" and changed the value from 44100 to 48000. Without really understanding how/why, this immediately fixed the problem. The odd thing is that when I went back into the settings, unchecked the box and changed the value back to 44100 (ie, reverted the settings to how they were initially), the problem did not recur, playback remained fine.
So I guess this is a bug, or some sort of obscure OS X/Reaper interface issue...? Anyway, this might help you if sample rate isn't something you've tried altering yet (I understand the frustration you're probably feeling)...