r/Buick

I drive a '03 Buick Century. It is a great car and I've had it for nearly seven year. It only has 130k miles and has had no accidents.
Last August, I had a coolant leak (it was coming from the right side of the engine block near the timing belt—I don't remember the part name). I promptly got my car serviced and the leak repaired. However, as I was pulling out of the auto shop parking lot my car misfires and it has been doing this ever since. Interestingly enough, the misfiring does not trigger a code. Moreover, when driving my usual route around town, my car almost always misfires in the same locations.
Over the last year, I have replaced everything you can possibly think of that could potentially caused this issue or even contribute to it. As I am not a car guy, I can't remember every part that was replaced or examined, but it was exhaustive.
I did have the ignition module replaced and had zero issue for an entire week, but then on my way to work one day my car stalled 8 times on the ten minute drive. I was then told it was likely a defective module and had it replaced again. My car never stalled again, but the misfiring almost immediately returned.
Then about seven months ago, a new issue arose: after long drives on the highway, my car starts to idle high, overheat, and push through the break. Target idle is around 750 RPMs, but when it idles high it is idling at an excess of 2300 RPMs, yet even this won't trigger a code; I have even replaced both my computer and dash cluster, but to no avail.
I know this sounds like a vacuum hose leak, but smoke tests haven't revealed any leaks. The oddest part, if I pull off the road and shut off my car and immediately start it again, it stops idling high.
My mechanic (who happens to my father) can't seem to diagnose my car and is at a loss for words.
I can try to provide more details in the comments if needed.

looking for recommendations on an aftermarket rpm gauge and any install tips for wiring