I take it that it is the 80s BM2000. Sounds like it is the protection circuit working. I would check the speakers cables and make sure that there is no short or poor connection. Martin is obviously wondering about the load of the speakers - if they are both very difficult to drive, the amplifier will try to protect itself. It probably needs a service - new capacitors and some variable resistors - they do gradually go out of spec and may well cause symptoms as you describe.