i recentlt bought 91rs with a 3.1 mpfi w/158k.been running fair but rough idle.i did basic tune/tps/iac/o2/ still rough idel.
i added a bottle of valvoline fuel system cleaner to full tank of reg gas.
drove it for 20 miles on back roads (30-50mph) no problem.
next morning i hit the highway (65-70mph) 7 miles on an up hill grade it lost power then quickly recovered,lost it again and quickly recovered then the third time lost power and i had to get off because it would only go 25-30 mph.if i pressed the accelorater more than 1/4 way it really choaked out elt oof it would run ..but slow.
drove like that for 5 mile toward home then all the sudden full power came again.
i drove the crap out of it for 20-30 more miles no prob.
next day another 20-30 miles no problems..
could this have been some dirt broken loose by the cleaner creating a monentary problem or is this my fuel pump starting to fail...

 

Sounds exactly like a failing fuel pump.