That sounds really similar to something i mentioned a few pages back. Is it immediately after you back pedal for a second that you lose traction and then it catches again?
I went and spoke to my lbs on my lunch the other day, and they reckoned it was probably due to the freehub getting gunked up. Which would make sense given i've rode the bike all winter and it's all farm roads round here, so pretty mucky. Even though i clean it after every ride, my bike isn't feeling particularly smooth to ride any more.
Anyway, mine's going in for a full service next tuesday (front and rear hubs and bottom bracket stripped down and re-greased, new gear cables, new chain, general tune up etc) so i'll see what they say once they've taken a look at it and let you know.