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Sadly a on GMs a P0300 could be just one cyl misfiring, or all 4.. they like to be vague like that.
Any chance you have a scanner that plots live misfire data?? That is a godsend to narrow it down to one cyl, or check if it's effecting all 4.
I'm gonna go ahead and say almost for sure NOT a clogged cat, though. We tried to replicate a clogged cat vacuum test (vacuum drops as exhaust backs up) and plugged the tailpipe with a stopper that had a pencil sized hole in it, and the car ran perfectly even with that huge restriction. I'd assume to show any issue at idle it'd have to be almost 100% clogged and then it'd do more than stumble every so often.
With that said, do a vacuum test on it and see what the reading is. If you start it and it starts at 18" and drops as it idles.. then maybe a clogged cat.. You could almost use a pressure gauge to check exhaust back-pressure (screws in where the O2sensor goes).. should be under 4 psi or so IIRC.
I'd almost suspect a vacuum leak.. does the stumble go away off of idle?
If you have live data, look at STFT.. Is it high and idle and regular revved up? If so.. vacuum leak. Find it by smoking the manifold.
If those are dead ends, def seeing the other live data feeds would be a great start.
Also, what caboose said.. those boot things are crap. Give those a check.
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