hmm, it could be a number of things. the only way to really tell these things is to try and isolate the problem. i can give you a few steps to try and find the problem. if you do these in order you may be able to isolate where it is.
1) Check all the wiring in your guitar. jam a different amp for a few hours.
2) Try a different cable.
3) plug the line out of your amp into a computer and play for a bit. if the problem is still there, then the problem is in or before your preamp stages. if its not, the problem is in your power stages.
4) different speaker cable to the cab.