Hey guys got an 06 buick lacrosse cxl with the 3.8L V6 engine and about 140,000 miles. I've got a p300 misfire code. Well one that reads as current and one as pending, not sure if that makes a difference.
One day while driving to pick up my girlfriend it started vibrating pretty violently. It was right before a light so I threw it in park for a moment, it stopped, and continued to be fine after starting to drive again. Then about 10 minutes later after I picked up my girlfriend it started vibrating again. I unfortunately was leaving for a weekend trip that same day so had to leave it for the weekend at my moms (who was fortunately a block from where it started). Her fiance found the oil was over a quart and brought it down. I thought it had helped since it was a lot calmer when I got to it again and started it but as I backed out the driveway it started vibrating again. I decided to just drive it home anyway and that's when the misfire codes and check engine started flashing. Since then, by a couple peoples recommendations:

• I have replaced the spark plugs and wires
• put some fuel system treatment/gumout in my gas tank
• took it to an exhaust/muffler shops since somebody suggested maybe it started from a clog in that but the people there said it was mostly fine other than the misfiring beating it up but they did replace my resonator while I had it there.

It's seems a little calmer now since doing all that and driving it around a bit, probably placebo. Current status:

• It doesn't shake in park but does in drive or reverse when not moving/very slowly but pretty much goes away when driving it.
• My performance is tanked, the CEL blinks when going about 30-40 (i'm not taking it on the highway but that's my towns main roads).
• I took a cursory glance at tubes and hoses and didn't see any obvious leaks, tears, etc but I don't have a smoker, however I occasionally smell gas and once when opening the hood while it ran I definitely smelled some gas waft at me.
• On start up the engine has a "dip" in it then comes "back up" (Think starts fine, engine powers down a bit, then back up)
• The biggest detail to me as of now is that when I started it this morning it felt like nothing had ever happened to it. It started up no dip, no shake in any gear, acceleration and performance was pretty much like before, but once I drove a few blocks in started acting up again. This is the earliest i've started it since the issues, before the sun was even up, so it was the coldest start its had, wondering if that is relevant.
• I'll also note the only other issues with the car is that the transmission definitely isn't the finest and can sometimes shift poorly and i get occasional weird electric issues (car thinks i don't have my seat on so it flashes that light, radio used to act up by displaying inproperly or not turning on at all but it's been a while since that).

Any help or recommendations would be appreciated. I feel pretty confident in doing repairs, i've done a handful fairly well, I'm just not a good troubleshooter. Unfortunately I've already put a decent amount of money in this car in brakes, struts, AC, a couple other odds and ends etc over the last 6 months and don't know if it's somethings very expensive that a mechanic will need to do i'll want to spend the money. So here's to hoping it's something I can do i'm just missing lol.