whats the problem with mt peugeot 206 1.1 fever?

the engine management light came on had it tested said it was 3rd injector changed that also had a new power pack still not firing on all 4.had another test done said it was 3 n 4 valves changed them and head gasket still loses power.also you turn heater on it loses power and the revs drop to almost stalling the car.