Problem with my Marshall Amp?

I have a 100watt Marshall Valvestate VS100 amp. Lately a strange problem has been occuring. During play she makes a popping noise then becomes barely audible. I turn it off and it makes a loud pop then turn it back on and it works fine for about 2 mins then repeats the problem. Lately ive been playing in really hot conditions and a mate suggested it may be thermal overload. I dont see why this would effect a sturdy constructed amp. Any smart amp people have any idea?