Undetermined Problem

I have a second hand 1993 Volvo 940 Turbo. The car running fine when I bought it but when the radiator was damaged due to overheat, some of the part went malfunctioning, most especially the idle gauge, the pin is vibrating everytime on park and stop position. To fix the problem, I replaced the water pump, hoses, fuel filter, air filter, catalyric converter, distributor, engine coolant temperature sensor and a second hand idle air control valve. But the proble is still existing. A week ago, when I step on the gas, my car has no power, stop and go motion so I have to step on the gas pedal very hard to activate the turbo after a minute the speed is accelarating. Today, my car stop at the stop light but the engine shuts down so I started the engine again. On the other stop light, the engine is running while on stop motion but when the green signal appears and I step on the gas, the car didn't move a bit so I step on the gas five times, then it move with less power. I already asked the dealer but no resolution. I am desperate, kindly help me troubleshoot the problem. Your immediate on this problem will be highly appreciated. Thanks.