Yeah I would have thought they'd be designed to take such long hours of use (studio producers and the like can run 8+ hour sessions) so it would be alright to run for at least a couple of hours. And I've not had any problems with any of my other equipment like you suggest. I have since switched to a power strip from the main wall outlet, which obviously hasn't solved the problem. I run two items off that power strip, but it can't be overloaded surely? The monitors do eat up a lot more power but still, it wouldn't be an issue.
The courier will be coming tomorrow to pick them up so it can be sent for repair, I'm hoping they can investigate and find its a dodgy solder or something. Otherwise I'd have to look at my electrics and reconsider if its got enough to power multiple objects in the room.
At the moment I run 2 x guitar amps, the monitors (when they were working), a pedal and in the evenings a lamp. One wall outlet has a 4 way which powers 1 amp and the lamp. The other on the far side of the room powers the monitors, the other amp and the pedal. Can't be a lack of power to keep everything running?