I have a 90 GS and when first started cold the engine runs to about 1500 RPM, once warm it settles down to around 750 RPM. Also while driving at a constant throttle under light load the engine seems to surge at around 3000 RPM, you can actually feel the car speed up and slow down. Tuned up recently and problems did not change. Any thoughts?

 

Check you fast idel control valve.