Help ! Buzzing on RD112 100W combo

I have been using this amp for a while and it has just developed this fault.
When I first start using the amp at high volumes it is fine, then after around 30 to 45 mins, it develops a loud background hum. The only way to clear it is to turn the amp off. It then re-starts for about 2 or 3 mins and the hum begins again. The fault does not occur if I just leave the amp on with no input signal, only when I am driving the amp pretty hard. What do you suspect? I am about to change both the 6CA7's, they look original fits.
You kind help please. You can email me on gaudoin4460@yahoo.com. If you know a good valve amp engineer in the Beds, Bucks, Herts area of the UK, please let me know.
Thanks
Jim