I have an old (1976) 100W Marshall MKII head that, needless to say, is way too loud for most of the situations I use it in. I wanted to be able to get that cranked tube sound, so I purchased a Marshall Powerbrake off of Ebay. It works fine for a while, but then will randomly cut my amp off for a few seconds and then cut back on again. It seems to me that it may be overheating. A few people I talked to said they don't recommend using attenuators, but I'm hoping I can get mine examined and the problem can be fixed. Any suggestions?