Sound cutting out/crackling. Ampeg 3Pro.

[goingdownslow]

I have a fault on my SVT 3pro that is similar to what you would get with dirty in/out sockets. I have cleaned the contacts on the sockets thoroughly, even tried linking them with a patch lead but to no avail. I know it is on the power amp section as I have tried different pre/power amp permutations. It does not always do it and it seems to do it more on low volume. A quick flick of the volume control up and down cures it for a while. It is not the bias.
There is one more set of contacts, and I have had a problem with this on a hifi amp which was cured by cleaning, and that is the speaker relay. 
Has anyone had this problem with a speaker relay?

It looks a bit fiddly to get at and pop the cover off but I will attempt it If I can't locate the problem elsewhere. 

Edited 3 hours ago by goingdownslow

[lemmywinks]

If the turning the volume knob makes a difference (albeit temporarily) then some contact cleaner in that potentiometer would be my first port of call.