I own a Markbass F1 bass head and I think I am having problems with it. When I am playing on the gig, i tend to lose power as the night goes on. I find myself turning the amp louder and louder as the night goes on. When I first turn it on, I hear the fan spinning and then it makes a churning noise off and on for about 2 minutes. Please help and let me know what I should do. This is a fantastic bass head and I would love to have it fixed. Thanks!