My guess from a distance is it's your spark plug wires. Or the plugs. Inspect the plugs for
carbon tracing on the porcelain. Check placement of the wires and the distance between them.
I'd replace the wires with AC Delco Professional (Gold) or whatever they call the higher
level of replacement with the thicker insulation. Be sure the wires are routed as shown in
pictures of the engines which keeps the wires away from others that might crossfire
with the spark or waste spark in the wire.
Personally I'd take off the metal shields on the rear plugs at least temporarily. At one
time I thought I was getting a spark down it.
I had this in different ways on all 3 of my leSabres. When you're going 45-55 cruising
the torque converter is locked up at 99% and the engine revs are slow. The fuel is
leaned out for economy. When you step on it a little for a slight upgrade, e.g., the TCC
stayed locked and the revs don't increase much, but the engine attempts to speed up
the car with a mixture that's requiring much higher voltages. The wires fail if there's an
internal gap in the carbon fibre or insulation problem.
If you disengage the TCC by keeping the accelerator the same but touching the brake
with your left foot, the car will accelerate through the same speed range with no misfire
is my experience. I also tried going slower with the transmission in 3rd and having the
engine rpms the same as the troublesome rpms in 4th gear. Lower gearing means less
demand on the ignition spark and I could not replicate the misfire at the same engine
speed. So I concluded it's just the engine rpm is kept too slow for gas mileage.
On my 93 leSabre after a few years, Buick had a replacement chip that changed the
speed of the lockup on the TCC they had then to a higher speed, which eliminated
a feeling uneven firing in the engine in 4th with TCC fully locked up. There may have been
other changes as well, but the lockup speed was the one I could measure.
