Recently, I had a motor replacement done due to a cracked block. Its been about 3 months now since I've had this done and I notice that every time I apply the brakes at a stop and upon releasing the brakes, my HHR will shut off. It doesn't do it all of the time, but it does happen often.

 

Welcome to the site, after 3 months, I doubt the shop will fix this for free, sounds like a loose ground wire or the fuse box halves of the underhood fuse box are loose. Take the lid off and press down on all the fuses and relays, might cure your problem. If not come back and let us know.

 

From the description, it may be a vacuum leak. I would check the brake booster and the vacuum line going to it.

 

The intake manifold could be cracked where the vacuum hose connects

 

Wouldn't a vaccum leak cause the engine to stall upon applying the brakes? OP says it stalls upon releasing the brakes.

 

Depends. On applying the brakes it uses the vacuum that is already stored in the booster reservoir. When releasing the brakes, it allows the vacuum to pull again and replenish the reservoir ?
Just guessing on this problem. Thought it would be a place to check since the only connection between the engine and brakes is the vacuum supply.

 

Hmmm, makes sense.