I have a Marshall DSL401 40W combo. Recently I've had to unplug the amp's reverb unit because after a few seconds of playing the reverb becomes fully engaged no matter what position the reverb knob on the front panel is in. I can still adjust the reverb level with the knob but after a few more seconds it becomes fully engaged again. I've searched multiple forums/websites but couldn't find anyone who had this problem. Anyone here have a solution?