Mine does the same thing and the same thing when stopped. (surging) Please repost what the results were if any. 08 2LT 29k, same driving scenario and symptom - tach bounces up and down with the surge right at 30- 40. It acts like it's trying to find the right gear to be in.

 

Not sure if this helps, but I had a simular issue on an Eclipse a few years back. It turned out to be the throttle linkage out of adjustment. Somehow it confused the ECM.

 

My car does not have this problem. But .. As for the up / down the engine speed - my experiences.
At the weekend I cleaned the Throttle Body. There has accumulated oil sludge and soot. Cleanup was successful. After that, increased the engine speed at idle. When driving speed have also changed. The fact that the HHR have "adaptive Throttle Body", Throttle has memory. Throttle Body learned to overcome the resistance of the mud. When the dirt was gone, the habit remained.
Treatment of the problem by - Reset the accumulated memory, reprogramming. All - in a specialized service center.
I'm no expert. But I think your problem - ECM. Cleaning, reset the memory reprogramming.

 

Though the HHR seems to run fine, I did run some injection cleaner to see if it might make a difference. None. I will take the car in next week to have it looked at. 46500 mi.

 

Update: Sorry I forgot to post back, the problem ended up being a bad trans control module on my 08 2LT 2.4...

 

Ballpark figure on cost?

 

Did a you get a CEL? I haven't and the surging is getting more pronounced despite periodic and recent use of fuel injection cleaner. Took it by the dealer today and they gave me song and dance. I was told if there isn't a code, there is little they can do. They will be happy to do a scan for $100, and suggested a fuel injection cleaning, but no guarantees. Good grief.

 

Because of an issue that I brought on myself, I had the HHR towed to the dealer. They had to do a scan because I was getting a REDUCED power message and the car was in limp mode. A piece of rubber somehow found its way down into the throttle body when I changed the air filter. grrr Throttle body cleaned, idle relearn, etc. I mentioned that I was having a subtle surging issue, and the tech felt that even the slightest dirt build up in the throttle body might cause surging and cleaning it would address the issue. It didn't. I thought that the surging issue might be caused by a dirty air filter. Even though I changed it a year ago, I thought it might be dirty as we a have had a lot of road construction that created a lot of dust that I drove thru daily. A call to the dealer for a follow up will be done next week.

 

Just a note about this......

The tech.'s at GM Engineering told me several times:

If you open the air cleaner to check the filter, clean it well before reinstalling. DO NOT MOVE THE FILTER UNLESS YOUR INTENT IS TO REPLACE IT. Other words, if you remove the filter to check it....just replace it after cleaning the entire air cleaner assembly. Seemed like good advise, so I always did this. Makes sense that moving the filter, itself, COULD dislodge dust, etc. into the bowl of the carb, intake, etc.

 

Good advice. I have worked on cars for years and am very careful, or so I thought. Hard lesson learned, and hopefully others will benefit from my mistake.