I had a problem with my Markbass 1 x 12" combo which bailed on me during a gig. We ended up getting an Ashdown amp (my old brand) instead, which saved the night. I use the combo for inner city gigs (bars and such) because it fits perfectly well on my bicycle (yes, I live in the Netherlands) and it has a great sound/weight ratio. The shopkeeper from whom I had purchased the amp sent it to a repair guy who changed (welded) some wiring on the printed circuit board, but after a few months the same problem occurred again. Very annoying. The power goes off and on now. It's a bit of a gamble. If it hadn't been for the large amount of money I spent on the amp, I would be looking for a new one, right now. Now I will need to go back to the repair guy and cross my fingers.
I hope you get your problem resolved. I've always found Markbass to be very reliable. I have a CMD 112P combo which is probably the one you are talking about. I've used it with a Markbass NY121 extension on many, many gigs without fault or problem. Perhaps you just got a bad one.
Lol, I've almost the reverse problem. My Ashdown Perfect 10 cuts out completely for about 10-15 seconds when being driven hard, so I'm looking at buying a Markbass 1x12 combo ('Combo Head 2'), which is lighter, smaller & more powerful. However, maybe I should get the 2 x 10 combo ( evidently their best selling combo ), which is 500W as opposed to the 1 x 12's 400W ? Any opinions greatly appreciated. ps. I'm in Queensland, Australia, at the tail-end of summer, so maybe the Ashdowns have a thermal cut-out ?
Try posting in the Markbass club. This may be a problem that a few others have had, and they have discovered what the issue is. Not trying to run you away from here, but there are some very knowledgeable, long-term, Markbass users that may be able to help you.