I had similar problems with my frst, i tracked it down to the throttle body, 10 years of crap built up in the base idle screw, i removed throttle & then removed the base idle screw from the throttle.
Soaked the throttle in petrol (any cleaning spirit will do) for a few hours, cleaned the screw up (even used a bit of T-cut on it) & cleaned the base idle hole with petrol & a pipe cleaner.
Look it up in your haynes manual before you remove the throttle, you've got to do something to set the idle afterwards & i can't remember what it is.