2000 2.4L Mid Range RPM Fluctuation

Hoping I can get some answers here. First off, some details about my car:
2000 Galant DE 2.4L 4G64 currently at ~230,000 mi
Timing belt last replaced at ~190,000 mi with a Gates timing component kit from Rock Auto (100,000 mi before that using Mitsubishi dealer service)
Rostra Cruise control kit installed at 170,000 mi (set up as throttle cable pull)

As the title states, I am getting a weird RPM fluctuation. It happens in between 2500 and 3000 rpm (2750ish). If I put the car in neutral and rev it up to 2750, the RPM will fluctuate from 2500 to 3000 and the car will shake a little. When I'm driving and the car decelerates from about 70 MPH (~2900 RPM) to 65 MPH (~2500 RPM), I can feel the whole car jerk back and forth with some force. It's worse when coasting downhill. Even with the cruise control off it does this, although it seems worse when it's on. Any RPMs outside that range and the car is fine. What could be causing this? This started after I had a shop put on the timing belt kit.

Are there any check engine lights on? The only thing near the timing belt that might contribute to this is the crank position sensor, but I don't think that's very likely. If it were bad, that would probably affect you at all RPMs.

If you put it under heavy load (aka floor it in gear) at that RPM, will it misfire or jerk? Or does it only do it when you're coasting/decelerating?

If you want to eliminate the throttle, try revving it from under the hood (in neutral, of course). Move the throttle by hand to rev it to ~2700RPM and see if it feels like anything is sticking.

Is it possible to disconnect the cruise control temporarily? That might be safer ;-)

No check engine lights at all. This only happens when coasting/decelerating. Tried manually moving the throttle and it was returning freely. Haven't tried disconnecting the cruise control. Anything else I can check?

I had some weird RPM issues also after a timing belt job once, and found out that it was because the balance shaft needed to be rotated once more. To get an idea of what I mean take a look at this video -

https://www.youtube.com/watch?v=UtTcRru-C-U

Another thing to watch for is, when the engine is cold, the RPMS seem higher than normal and the vibrations are quite noticeable.

That explains a couple shitty timing jobs from past shops in places I used to live. Thank you! :P

NickF, in response to your comment: The cruise control is the big variable I'd like to eliminate. Can you disengage the cruise control cable from the throttle body? I think that's a fairly simply procedure, and hopefully shouldn't require messing around with the aftermarket unit too much.

Are there any other odd symptoms? Does the engine take a while to catch when you start it? Any indications that you've got compression issues or valve trouble?

The fact that it happens only on coasting/decelerating seems suspicious. In theory, the injectors should shut off completely when coasting with zero throttle. If the throttle is opening, or you're getting some fuel that shouldn't be in there, it may explain some of the issue.

Either way, I'd like to eliminate the cruise control/throttle as a potential factor.