Not sure if this will help, but mine did this after I put a recon. head on. The idle was set too low.
Get the car to normal operating temp, switch off all electrics and disconnect the idle control valve (white connector on top of the unit to which the throttle cable goes to).

Then screw/unscrew the white plastic nut through which the throttle cable goes until you get the car to idle at 850/900 rpm. I find it's worth giving the throttle pedal a little push after every adjustment. Note, a tiny amount of change on the plastic thingy can make a big difference so go slow.

If that doesn't cure it, I have no idea - sorry.