I have a JCM 90 50W Hi-Gain Dual Reverb with GrooveTubes biased at by a Marshal qualified tech years ago. This Amp periodically drops volume and I have never gotten it to stop for more than a few weeks. It appears once the AMP is used quite a bit it wont do this but I use it less and less now that I have less time to play Nothing is in the effects loop and I just had an SG plugged into the front end of the Amp with the volume on about 5 gain cranked but it will do this on the clean channel as well. The power mode is set to high The foot switch wont work and it's not plugged in so the only cables are power, speaker and guitar. the speaker cab is a 1968 Marshall 4x12 set to 8 Ohm, amp is set to 8 as well. The power in the house is clean, the cables are good and working, Ive had this happen with more than one speaker cable. I've had the Amp for about 20 years and I'm wondering if I should just cut my losses and get something smaller. Also about how long should I warm this up before I play? Apologies if this is the wrong forum, this is my first post.