Loop Record has always been weird for me and just recently corrupted a project. There were many other factors in play though that I won't go in to here.
Right click your Record Button in the Transport Module and be sure you are in Comp Mode. Up your buffers (interface and HDD read/write cache) BIG time before beginning Fast Comping (Fast Comp is one of the major causes of dropouts on my system.. but again other factors are in play). Definitely study up thoroughly on comping procedures and keep a cheat sheet by your keyboard... and don't just go by those Cake TV vids. They are great but check out the manual... there is more to it and doing things WITHOUT entering fast comp isn't quite as strenuous on the system. Actually the fast comp stuff relies on the old audition feature which again has always caused weird crap for me (like dropouts).
Try to use as few splits as possible too. This seems to help because I think Sonar readies itself to automatically play any and all combinations so the more splits you toss into your comping set up the more resources get hogged.
I could be wrong on that and this is all kind of stream of consciousness speculation/advice but I use comping all the time. It is still a bit mysterious to me as far as its quirks and needs but I've managed to bend it to my will if I don't push it too hard.
Maybe not helpful... but maybe it is.
Good luck.