[roaddirt]

Can anybody help with this one?

Vehicle: 1999 Buick Century
Mileage: 40K
Engine: 3100 V6
Transmission: automatic 4T60E w/vacuum modulator

The car has been checked by my regular mechanic twice, a transmission shop and a Buick dealer service shop in Richmond, VA with no success.

The problem is more noticeable when car warms up.
Once I reach wanted cruising speed of 35 mph or above (and the car changes to third or fourth gear), the car has a poor response on a slowly, steady press on gas pedal.

For a second it seems that RPM sound goes up and then it drops down with dull sound ( the car has no RPM gauge on dashboard). At that point I keep pressing pedal slowly but car does not accelerate or needs eternity to speed up.
Sometimes I do not even feel that RPM goes up even for that second.

At that point,
If I release and press pedal lightly once or twice, it will eventually “pick-up” and response.
If press faster or harder, it will downshift and speed up like a champion but I do not want to race.

Air and fuel filters, transmission fluid and PCV valve has been replaced - problem persisted.
So far I tried test-swapping used TPS, MAF, MAP – problem persisted
Fuel pressure within specs (40 psi in idle) and fuel pressure regulator holds vacuum.
Spark plugs and wires are original but they should be good at 40K, (GM says the should last up to 100K).
Poured a bottle of injectors cleaner in tank - problem persisted
Replaced vacuum modulator with aftermarket part – slightly better but problem persists.
No SES light came, no malfunction codes set.

I love the car, it has low mileage and hopefully should give me years of service but I am desperate now.

Any help would be greatly appreciated.

Roaddirt