Rover 200 loss of power

basically the cars power just seems to go. there's no pattern in when it happens, but suddenly the revs drop straight down to zero and the accelerator doesn't work. the clutch, gears,brakes all still work. i have called the rac out twice in 2 days with this problem, each time thinking it was fixed. the first time the guy traced it to a socket to do with the fuel pump and fuel gauge. the 2nd time, the guy replaced the engine management fuse which had blown. when the loss of power happens, the car will start up straight away again and will quite happily tick over but as soon as the revs are put under pressure it will stop again. any help will be much appriciated!!