Problem with my marshall jcm2000 tsl 122 100W Combo, Reverb on full after warm up

I love this amp, but I've yet to have anyone really service this and the reverb is going crazy.

when the amp warms up the reverb bleeds full on no matter what the control settings are even feeds back in the clean channel and I have to put it on standby to get it to stop.

what could be causing this?
is this a major fix?
any ideas