Frankco

ive got some problem with my scooby...car is running sweet most of the time but once a while revs are playing up and down thats about 100 up and 100 down and then is running fine for a bit again... when is playing up im getting slight valve rattle(sounds like valve rattle but im not sure could be a coil pack playing up)and then when it settles is quiet again...and the main problem is when i accelerate car goes fine some times but oter time im geting massive back fire and i dont have any boost from the turbo after that until ive switch the engine off and let it stand for a bit... Any ideas what could cose that...
thanks