I have a '83 built F2B that on ch1 after playing for a while cuts out then after a while comes back sometimes at lower volume and/or distorted then cuts out. Ch2 is fine I have changed the valves several times but I still have the problem.
Does anyone have any suggestions?

Probably just need to clean the mono output jack. You'll find all the details here. If you perform this mechanical cleaning and the problem persists, the mono jack may have to be replaced.

Thanks Mica I will give it a try