KT88 Intermittent problem

[A.N. Beal]

Dear Folks,
Can anyone advise on a strange intermittent problem in my KT88 XL amplifier? Most of the time it works (and sounds) fine but sometimes (most commonly at switch on but occasionally after running for several hours) the left channel is very quiet (but not silent) and the right channel is louder than usual. There is no hum on either channel and there are no hisses, crackles or other 'problem-type' noises.
I find that if I turn the volume up and down quickly a few times, the problem is solved: the right channel drops to normal volume and the left channel comes up to normal volume and it then runs perfectly for the remainder of the listening session. However this 'cure' only works if there is a signal going through - turning the volume control up and down without a signal going through does not have any effect.
I assume that it is probably a slight dry joint or poor connection somewhere and expect that I shall have to open it up some day when I have time and poke about to try to find and fix this. However I would very much appreciate guidance on the most likely place(s) to look to find it. Based on the above description of symptoms, has anyone got helpful suggestions?
Regards,
Alasdair