ignition or lack of fuel.
I had lack of fuel problem in my mustang when it would (atempt) to get into the power. The computer starts to adjust timing to give more power and prevent detonation. However the jets were wrong (not rich enough) and the car would ping and the computer would kill the timing resulting in a nasty backfire and other un fun stuff.

I know it is not Cosworth, but may somehow help. I am not all to familiar with the coswrorths.
TF