Haven't had the time to check the manifold vaccuum.

O2 sensor is less than 1 year old. Car passed inspection in Nov with flying colors. I would thinkg that clogged or restricted catalytic converter would have adverse effects on the emissions.

New plugs, wires, cap & rotor. Coil is original. No exhaust leaks.

Additionally, I believe a clogged converter's restriction on exhaust flow will not differ based on car speed, but on engine RPM. I think I noted in my post that the car pulls like gangbusters through 1st and 2nd. It's at freeway speeds in 3rd that I see the fall-off in acceleration.

If the converter were plugging up, I'd see the same "nosedive" regardless of the gear.

I'm more inclined to think this is a fuel issue. In the lower gears there's less load on the motor because of the mechanical advantage provided by the lower gear ratios. Additionally, the car spends far less time in each of those gears - especially 1st.

In the higher gears, there's a lot working against the motor - wind resistance (50+ mph), and the lower mechanical advantage of the higher gear ratio means the motor works harder for longer at higher revs.

If this problem is fuel-related, then there would ordinarily be indications because of lean-mixture induced detonation. The knock sensors will likely prevent me from ever noticing the problem - the computer will prevent pinging by retarding the timing - which will further exaggerate the acceleration fall-off.

The car is completely driveable and gets good fuel mileage at 75 (around 22.5-23.5). It's just when I'm flogging the snot out of it to pass on a 2-lane road, or trying to move around some dimwit driving 55 in the fast lane when traffic is flowing 70+ that I notice the problem.

Far as I know, the fuel pumps are original and the car has 121K miles.

I need to get a fuel pressure gauge so I can check for pressure fall-off when the car is misbehaving.

But I'll still check the manifold vaccuum. Couldn't hurt.