I've been having this problem for a few years, but never took the time to do anything about it.  I have an Orange Rocker 30 and it plays fine on both clean and dirty for a small period of time (usually around ten minutes) before randomly cutting to a to a very loud feedback sound.  If I don't use it for a while I can play it for around ten minutes again, but after the feedback comes in it comes back almost constantly.  Any ideas on what the problem is/how much it would cost to fix it?  Many Thanks.