My Solstice says "hi everyone." Moving on.
I've had some really rough starting over the past month or so. I hadn't had time to really look at it until a few days ago, when I noticed the downstream O2 sensor harness had been cut, I'm assuming by some kind of road debris, though I'm not sure how.
Yesterday I replaced the sensor and harness connector (which had melted while hanging freely next to the turbo / exhaust manifold), and, lo and behold, my CEL disappeared and the car starts much more smoothly, though not perfect.
The other (possibly-related) problem is that, under WOT, the car hesitates to the point where it feels like a fuel cut. It's intermittent, but frequent.
While I was changing the sensor, I noticed that the gasket between the turbo and the exhaust manifold had blown. There's about a 2mm space between the turbo flange and the manifold on one corner, and a piece of the gasket is sticking out.
I've ordered a new gasket, planning to install it this weekend.
Can a large exhaust leak cause this sort of hesitation? It hesitates, surges, sounds really bad and I have to lift, then gradually accelerate to get it to stop. Not fun, a bit dangerous the first time it happened.
It may be worth nothing that this just started a few days ago, before I changed the O2 sensor. I believe the car was running rich before that (makes sense as the ECU couldn't read anything from the downstream sensor), may have messed up the plugs. Too many things to check! Out comes the service manual, diagnostic section. :/
Update the 2nd
Unplugging the battery for half an hour re: @Paulster2's suggestion appears (so far) to have addressed the rough starting issue, but the weird hesitation at WOT is still happening. I haven't tried the tape yet. I did, however, notice that my AFR gauge is reading about 10.0-10.2 when it's happening, which is typical with this tune (though it seems a little excessive to me). That leaves air and spark. I'll check the plugs as they seem to be the most likely culprit.