My Behringer (yes I know...) BX1200 120 watt combo is acting rather strange. I plug in the other day and after only 30 seconds or so, it just sorta dies. The preamp still works, all the lights come on, and I can play with phones, but the speaker won't work. So I check out the speaker, it's fine. I take it to a friend who has a good idea about how this kinda crap works, and when he plugged it in and turned it on, it worked. So I then take it home, plug in, still works fine. A little later, it dies again. I hit it, it works...for 30 seconds. Any ideas what the problem might be? Any help would be greatly appreciated.