I had exactly the same problem Nog and after buying new:- Chain x 2. Cassette x 3. 1 Cheap. 2 Expensive New cables x 1, had made no difference. And I was still forking out at LBS to have bits fitted. The last resort diagnosis was grit in the cable shifters. What rubbish. I brought bike home and after speaking to another LBS I bought a chainring. The problem has never returned.
My bike is a Specialized too. For the sake of £6 to £8 quid for a new chainring you can fit it yourself if, you have a crank extractor, its a doddle.
Split the chain and take it off the rear mech altogether THEN wrap it around the largest chain ring and largest sprocket then add two links. This is the correct method as recommended by shimano. I have used this method many times and does work
I received the BETD "strong" bush kit today (just finished fitting it). How different the bike feels without it rattling around the thing supplied by Mojo, I just hope it lasts! Anyway, As I have slagged Mojo off, I now need the name of someone who can service Fox equipment, any ideas guys?