1995 Vauxhall Omega stalling when I switch on rear window h

My car is stalling when I put it into drive/reverse. It may stall at traffic lights, if I restart it try again it doesnt stall, next set of lights repeat again. Also noticed revs drop with power steering, rear window heater, lights etc.(Anything that takes a lot of power)and sometimes will stall. It starts and runs fine.
have replaced crank sensor, air and fuel filters, vaccum pipes, cleaned out intake manifold, egr valve.