2001 Century surging and no power

I have a 2001 Buick Century, 3100 V6 engine with about 135,000 miles that has developed a problem. It "cuts out" or surges when you are normally accelerating and at cruising speed if you come to a hill or have to accelerate, and is very sluggish with no power. When you depress the accelerator past a certain point (not downshifting the transmission) when accelerating, the engine smooths out and accelerates well. Service engine soon light has not come on, no computer codes thrown.

Recent work to try to fix the problem: replaced intake manifold gasket (2 times), replaced coil, replaced fuel return valve on the fuel rail, new spark plugs, new spark plug wires, new fuel filter, new EGR valve. Catalytic converter checked and okay.

Buick Dealer garage diagnosed vehicle and said the problem was leaking intake manifold gasket. Replaced twice and did not help the problem at all. Any help or suggestions would be greatly appreciated. Thank you.

What brand spark plugs and igintion were installed?

It sounds like you could be getting what is called "chuggle" or "fishbite" which is the torque convertor clutch acting up. It may not be locking/unlocking properly or staying locked up.

When was the last time the tranny was serviced?

GM has TSB out on this problem and it mentions the throttle position sensor (TPS) possibly causing it not in all but some cases.

The car had the transmission/transaxle replaced with a new one by the dealer about 2 years/25,000 miles ago. Will check out the TPS. I suspected it but thought the computer would throw a code/SESL if the TPS was bad. The spark plugs and plug wires were installed by Buick dealer, assume they are AC/Delco. Your help is greatly appreciated!

Problem fixed. A local mechanic tested the pressure in the exhaust manifold and said that the catalytic converter was stopped up. Replaced with a new catalytic converter and the problem bit the dust. Runs like a new one. Thanks!