Misfire problems - But not the usual kind

Hello, I am at my wits end tearing my hair out over this. Been a mechanic for quite a few years now but I cant seem to figure this one out. Its my personal vehicle and time is limited to get it finished.
I have a 1999 GMC Jimmy 4.3
It has come a long ways since I first purchased it but three's something left I cant seem to figure out on my own. It didn't run when I got it, however with a lot of work its going now.
So far I have replaced:
-Spark plugs and wires. (Plugs AC Delco / Wires I believe are house brand since they were out of the ACDelco brand wires)
- Cap and rotor (both AC Delco)
- Fuel pump
- PCV valve
- Intake Air temp Sensor
- Catalytic Converter
- Water pump
- Cleaned the MAF very well and it works properly.
- Few other things.
I will note that when I got it, it had a blown head gasket. However I fixed that with two seperate rounds of Blue Devil Head Gasket sealer. One time to fix it, and the second time just to be sure.

Problem is that I cant get it registered because I cant get the check engine light to go off. Only thing left that comes up varies. Sometimes its either a P0300 (multiple cyl mifsire), or a P0302 (Cyl #2 misfire). But never at the same time, its always one or the other.
The Check engine light stays on solid, but then when I get up to around 30 to 40 mph the light will start flashing. I am leaning towards it possibly being the Spider fuel injector malfunctioning. Because besides that the damn thing runs perfect!
Any help or input would be much appreciated thank you!