I had the same problems with my 1993 C20XE Calibra, what years is yours?
I did the following
1) clean out air box and air pipe to throttle body, new panel filter and ensure all jubille clips are done up
2) buy a can or carb cleaner (£4) and clean the black diposts off inside the throttle body and butterfly flap.
3) also clean out the breather pipes, 2 come off the back of the rocker cover and 1 leads into the throat of the throttle body. Clena these both pipes out with carb cleaner
4) if you have time, clean the ISCV out as well
5) fit allback toghter, buy a bottle of STP cleaner and chuck into the fuel tank
The icing on the cake for me was when I ditched the cone filter and repalced it back with the air box and panel filter, it picked up better from stand still