Not unlike my experience, I often get the difficult ones...
The 2006 2.2 Automatic Amethyst HHR of mine (rebuilt salvage title car) that went throug the woods and got peppered with dents and a bent cradle, radiator, condenser, and front bumper cover. began missing and cutting out randomly about 2 months after I got it licensed and on the road. I fixed it this past summer and have been driving it this winter. If you flat floor it during one of these missing sessions it goes into reduced power mode and will soon be on the side of the road, unless you are doing more than 70mph...
Dealer found MAP sensor code set from a brand new MAP sensor - of course a symptomatic code - MAP would pick up misfire out of paramater from disrupted intake vacuum.... Its symptomatic not problematic... They though it was a throttle body "we put alot of them on". So, $485.00 later the whole scarade begins again 3 weeks later - same miss and cut out as before.
Its now with a mechanic known far and wide for solving the really tough cases. He cannot get it to miss at all.... figures, it stalled on me twice putting it on the trailer to take to him... He has spent quality time diagnosing and checking connections and is afraid that he may have "fixed" a bad connection the process - sans the learning...

Any Ideas?

I read all the other posts here and just wanted to start a thread on my mystery problem...

Oh the fun we have here in Michigan Winter !!!!!!!!!!!!

 

My mechanic finally got the cut out to happen after 4 days... With all his stuff hooked up it began to do it regularily for him. It is the MAP sensor circuit and he felt it was not the sensor and checked the wires 30 ways west of Sunday... There are only three players in this circuit - MAP sensor, wires and ECM.
So it was off to get another ECM
Why? The 5v referance voltage from the ECM dissapeared each time the miss occured. There must have been an intermittent open to ground inside the ECM.
Once the replacement ECM was in and we downloaded all the cals - reset it to the other modules etc... the problem stopped.
WWOO HHOOOO!

 

Good mechanic to have found that problem. Keep him around. :)

 

Good job glad to here it worked out ok.

 

Nice to know there are still techs willing and able to dig into an issue until they find the root of the problem.