Fender FM65 guitar amp trouble shooting

I have a Fender FM 65 guitar amp my buddy is using with an acoustic guitar. It sounds great for about 15 minutes or so till it warms up good. Then it begins to cut out, not totally but just kind of farting. I did replace the speaker with a 150w speaker and it helped but still not quite right. Any insite into this problem?

darn... posted a "quick reply" he other day but it disappeared.... sorry... check the soldering on the circuit board. chances are there's a "ring joint" on one of the solder pads. that's where the solder in the middle separates from the solder around it leaving a slight "ring" around it. as it heats up the copper and solder expand, breaking the connection. usually happens around the middle wire on transistors, but can be just about anywhere.

The solder connections on the input jacks are a common problem.