06 Buick Lacrosse 3.8L Surging/Bouncing RPMs when accelerating lightly

[th0mathy]

I've got an issue with my 2006 Buick LaCrosse where the RPM's surge anywhere between 1800 and 2200 RPMs. Sometimes the bounce will be +/- 100, while other times the engine will bounce a bit more aggressively, likely +/- 200 to 300 RPMs. As soon as I let off or put more pressure on the pedal, the issue disappears. This issue began almost immediately after I replaced my Air Filter, but I've checked it 1000 times to make sure it's the right size and that it's seated properly. I've even replaced it with another filter, but it didn't stop.

Now, I'm aware similar posts have been made, but the solutions I've found haven't worked. I cleaned my MAF, cleaned my EGR, replaced my Throttle Body, bought new spark plugs and wires, and tried a vacuum leak test with a can of Carb Cleaner, and nothing has shown up to be a problem.

Another thing that makes my problem unique is that this issue will stop sometimes, mostly after I disconnect the battery for a few hours. However, it'll start back up again after a few weeks of it driving completely normal. This is the part that has really stumped me.

As for data on my scan tool, the only thing I've identified that may be an issue is that my Fuel Trim seems to always be negative. It seems to be -5 at idle, and between -10 to -15 when driving. I'm not sure what's causing this, but I have no check engine lights indicating that there's any type of fuel problem. I've also checked my mass airflow sensor data, and it seems to be reading as you'd expect, but the data does bounce up and down when the engine and RPMs do. Otherwise, the data seems completely normal. I'm no mechanic, so there could be valuable data I'm missing, and I'd happily upload my data if someone here is willing to look at it.

I've been left completely stumped by this issue, and it seems to be becoming slowly more frequent as time does on. I'm not sure what my next step should be, and I'd greatly appreciate some help here