Loss of power

My hijet is a P reg 993cc petrol. Had no trouble with it since I got it 2 years ago, never let me down. Now it ticks over fine on the drive, but out on the road as soon as you try to drive up the slightest hill, it hasn't got the power to pull the skin off a rice pudding, accompanied by a noise that is difficult to describe, the best I can say is a low droning siren type noise. Got so bad today had to pull of the motorway as only doing 30 mph up a hill that I would normally sail up at 55 mph in top gear. Then when going the long way round home on the B roads, suddenly as if someone had flipped a switch all was OK. Did this a week ago for a few miles, then again for the last 2 days, Ok at the moment, my worry is when will it happen again. Anyone got any ideas what's wrong? Oil and water levels are fine, but she's running a bit hotter than normal but not overheating.

It could be many things but an easy one to check is that the electrical connectors on the fuel injectors are on I had the rear one come loose once no idea why but the noise could be it dropping onto two cylinders and the back to normal if the connections touch pop the passenger seat up you can just get on to the top to push down I seem to remember seeing a bit of orange or red that I hadn't noticed before and the connector covered it up.

Maybe the exhaust silencer has collapse in on itself?

Efi or Carb?

If its a carb one I'd say it might have a blocked jet,

If its efi I'd guess at maybe the battery connector in the boot which always fails, or the map sensor hose which apparently often splits and breaks, or the fuel pump might be faulty

Just some ideas really