I have a 4-speed automatic transmission in my '85 Firebird with the MPFI 2.8 litre V6. As soon as I hit highway speed the other day, the transmission started hunting between 3rd and 4th gears. The next thing I knew, I couldn't go more than 50 mph: pressing harder on the gas made me lose speed, backing off allowed an upshift and I could then maintain speed.

When I got to my destination, I put the car in Park, in which case I could rev the engine. When I was driving, I couldn't throttle without the engine bogging down.

Fuel filter changed: no difference.
Disconnected MAF sensor: no difference.
Checked timing: 11 degrees.
No codes stored in ECM after several short to medium trips.
EGR valve does not leak and moves freely.
TPS was fully tested and is OK.
Coil was checked with an ohmmeter and is OK.

Tested fuel pressure with an Actron CP7818:

46 PSI at idle and vacuum hose disconnected from fuel pressure regulator.
Pressure dips a smidgeon for a fraction of a second as I stab the gas pedal.
46 PSI at a variety of steady engine speeds (4000 RPM +).
38 PSI at idle with vacuum hose CONNECTED to fuel pressure regulator.
46 PSI at higher RPM with vacuum hose CONNECTED to fuel pressure regulator.
46 PSI under load: car in drive, left foot firmly on brake, and right foot giving gas to about 1500 RPM.
Ignition ON, engine OFF, fuel system pressurized to 42 PSI and crept down to 35 PSI after 4 minutes.
All this seems to be OK except for the slow depressurizing when engine is OFF.

I still take the car out to go to the corner store, but I use another vehicle for longer trips. It starts fine and runs fine up to 20 mph. After that, it's apparent that there's a serious problem. As I said, 50 mph is the current top speed, and getting there is painfully long. The transmission does hold its gears much longer than normal, but shifts firmly when it finally does shift. If I mash the gas at low speed, the engine will eventually rev quite high and tranny won't want to shift. At higher speed, say 30 mph, mashing the gas will bog down / stall the engine.

I plan to hook up the fuel pressure tester again, this time with the gage sticking out of the hood so I can see it while driving (thanks RB83L69).

Any ideas?

 

I suspect the cat is clogged. Try unhooking it and see if the problem goes away.

 

Today, I removed the catalytic converter and replaced it with a straight pipe. Problem solved!

The guts of the converter had broken up into about 4 large chunks and a pile of 'sand'. With the passing of the exhaust gases, one of these chunks got sucked into the rear exit hole of the converter and got stuck there in an almost sideways orientation, thereby blocking the exhaust flow.

So this repair fixed three problems:
1) loss of power (and I now have more than I originally had)
2) rattling noises
3) dangling exhaust

Thank you for everybody's help!