Bassman 5B6 strange hum

[apache]

Hi,

I built a fender bassman 5b6 about half a year ago. It worked fine until about two months ago when it suddenly started humming really loudly, and I had to turn the volume up really high to hear my guitar coming through. At first I thought the problem was the 6sc7 tube but when I got a replacement the problem was still there. I tried it yesterday and it had the same problem as usual, but then I tried it a few minutes later and the hum was mostly gone, but when I played there was still a really odd buzzing sound when I played notes and the amp lacked its usual clarity.
I'll have a go at checking all the voltages on the weekend, the fact that it sometimes works sort-of well and sometimes works horribly suggests that theres a connection problem but i couldn't find any.

Thanks