I got a Laney GC80 solid state amp from a friend for a steal at 20USD....he found it in the garbage but when I got it home, it fired up and was very nice..But now im starting to notice that every once in a while the amp sounds dull and when you start playing the volume crackles and fluxuates a little...and if u keep playing for like 30 minutes liek that...it will "warm up"....Any ideas on what this could be? Short of buying a Line 6 Spider 75w Amp head(because the speaker inside the amp is disconnectable through a wire from the head...although theyre built into the same cabinet....) Kind of confusing....Any ideas as to what this could be? I dont want to buy a new head to replace the one in there!!!!!

PS would the whole putting a wire between the send and return on the effects loop be a trouble shooting step?