The "couple miles down the road" concept screams EGR to me but by all means get a obd1 code reader.
My EGR works great but all parts have been replaced in the last few years. I was cruising to my cheap gas place today and the computer was saying 26-32mpg at 55mph with 3.73 gears.
Then when I got home I noticed about 8 feet of rub rash with something grey on the right side. I hope it buffs out.
1992 Tbird LX 5.0, 3.73 gears, 1995 Cobra intake, deleted air silencer, 2.5" mandrel bent dual exhaust, SCP shorty headers. Updated four-hole fuel injectors. SK shift kit in AOD. Vogtland 0.9" springs, Tokico and Bilstein shocks.