A dirty or faulty idle air valve can give that problem. If you remove it, it can be sprayed & soaked in carb cleaner or electrical cleaner to remove varnish & carbon.
Best not to spray it in place because of the risk of damaging the coating inside the throttlebody.
If the valve is your problem & this doesn't help, it will need replacing.

I have the same symptoms on my 88 right now, but only when the tanks are low - lopey idle etc - but fine with more than 1/4 full tanks (front or rear tank - makes no difference). Still trying to figure that out