Ok so here's the deal my 2001 blazer 4x4 has been missing for quite some time now and all ive been doing is changing parts and its becoming ridiculous So It has new AC delco plugs,wires, cap, rotor, new coil, all new vacum lines I also had the upper intake off to clean and check injector unit. It throws code p0300 Its missing in cylinders 2,6,and 5 only but I dont know what to do When I changed all the vaccum lines it ran great for a day with alittle bit of a slight miss but it kept getting better so figure it just needed to clean its self out. The next day got worse check engine light back on slight miss but only when idle when hot. Then the next day It goes back to being even worse now when i step on gas alittle to keep speed or to go it sputters and coughs like a mother I just cant figure this damn thing out I hope some one can help bc If not ill end up replacing the whole top end before it runs right ever again.

 

Had this on a broncos I had and it was an o2 sensor

 

had a buddy of mine with a p0302 and ended up being a couple things:
1 had a bad ignition module
2 ECM was bad due to trying to compinsate for #1
3 found a faulty injector plug connector..

his S-10 is running great now, 24 mpg!!!
and we did all the same things that you did...
some times its a lot cheaper to have it diagnosed even tho that could be
$100 bucks, but will save time and $$$ in the long run, then you can still
do the work

 

Cat.
Bang on it with a hammer, if it rattles, replace it.

 

One of my thoughts is, when you replaced the cap and rotor did you try to turn the distributor shaft? The gear can wear out (should before the cam cheaper and easier to get at)
That can cause alot of problems. Seems a little odd if that's it that it would only be 3 cly's though. It will turn a bit.

Does it start fine? Maybe a lower intake gasket leak.
You said you checked the injector assembly, do you know for sure the "injectors" themselves are firing fine?