you might want to check your EGR valve
operation. it works on two things at once, vacuum and exhaust backpressure. The vacuum part may be working correctly but the backpressure may be opening up the EGR because the spring is weak. Try driving it with the EGR vacuum line off and plugged. If it seems to be gone, replace the EGR valve. If you do need an EGR valve
get one from the dealer or an AC Delco one. I have found that some aftermarket valves don't work correctly for these 90's vehicles.