Erratic RPMs

[9c3]

1999/3.8L/60,000Miles

The car will start jerking back and forth and the RPMs will fluctuate up and down between 1800 and 2200. This only happens once in a while when I'm cruising at a constant speed of about 50mph, up a long hill or on a long stretch of flat road. It's not an acceleration thing as it only happens when the car has been in gear for a while. The jerking isn't violent, it feels more like the car rocking back and forth in Park, except it's pretty quick.

Reminds me of the erratic RPM fluctuation you get with a bad alternator. I'm not sure that that's the problem though. Fuel filter perhaps? I know that the 3.1 engines have a problem with the intake manifold gasket, do these as well?

It's not tripping the service engine light, so it makes me think that this is possibly gasket/filter related.

I should also mention that the odd time I will get an EVAP service engine code. It will stay on for a couple weeks then not come on for another month or so. However, I get the erratic RPM problem regardless of wheteher the light is on or not, that's why I think it's not EVAP-vacuum line related.

[cadgear]

Might be the TCC solenoid. My 99 3.1 surges around that range (60mph + in OD). Especially when you say "long stretches of road", that suggests to me that the TC is trying to enter lockup to save some gas. Happens to tons of 4T60E/4T65E transaxles.