Had the same issue with my old '93 +8, it turned out to be the stepper motor sticking.
+1.
Crankcase fumes are fed into the plenum chamber to be burnt in order to reduce emissions. This gums up the stepper motor. It is possible to clean the piston shaft of the stepper motor but the rest is a sealed unit and the bits that move the shaft can't be cleaned. Once it starts causing trouble you are best off changing it. Any problem with tick over is usually the stepper motor till proved otherwise!
