The car is a Kyosho inferno 7.5 sports RTR. Stock engine.

When i fireup the engine it starts going backwards ! If i hit the throttle, the car die. But if i let the car go warm, about 70-80 degrees Celsius, then hit the throttle, the engine makes a loud bang noise, and the starts to go the right way.
It have never happened before, but now it suddenly is doing this everytime i start it.

This cant be very good right ?
Anyone know whats making my engine do this, or what i should do to solve it ?
I have changed piston and sleeve at the start of this summer if that has anything to do with it.
Running at 15% nitro, stock glowplug.

Does the car perform better if i change the glow plug ?
Which plug should i use ? Anyone have a clue ?
And what is the maximum engine temperature ?

Thx in advance // Ivah =)