It certainly could be due to overheating. That amp is rated for speakers down to 4 ohms (one thing that I wanted to check on), so I don't think that's the issue. Have you noticed it getting hot? Is it in an enclosure that keeps it from getting good ventilation?
One way to check if it's due to overheating is to blow a fan over it while running it, and see if it shuts down as quickly (or at all). If keeping it cool solves the problem, then perhaps adding a small fan would be the ticket.
You might already have it, but you can get a copy of the owner's manual
here. It has some useful info.
Regarding amplifier suggestions, where are you located (country)?