P0300, Rough idle and sluggish.
 

I have a 2000 Blazer, my first vehicle and I'm not very knowledgeable about engines, but hoping to get some advice.
The engine has a hard time starting, usually have to pump/hold down the acceleration to get it started. While driving it uphill (I live at the top of the hill) the car is sluggish and very slow. Driving on a straight away it generally seems fine. While its in idle the car shakes.
I'm getting a code reading of P0300 (Random/Multiple cylinder misfire).

Recently (about 2 weeks ago) replaced intake manifold gasket and water pump pertaining to a leak/overheating issue. Got the leak issue first, the misfiring symptoms (rough idle, hard start and sluggish engine) developed quite a while afterwards.
6 months back had the fuel pump replaced and new air filter.
Not sure if any of that is important, but thought I'd list it anyways.

umm did you do the routine tune up--distributor cap, rotor, plugs, wires? If not then that could be the problem right there pertaining to the misfire. If you DID.. then what type of parts did you use? These vehicles are all about the Ac Delco brand. The aftermarket brands usually leave it having that code and symptoms.

Ive always been told "It doesn't matter if the guy your buying it off says he just did it 5 minutes before you arrived. You NEVER take their word on it."

And if you got it from a dealer.. then i suggest you take it back to the dealer and have them fix it since it should have some sort of warranty on it. BUT also pertaining to that.. i assume you DID buy it off a private dealer since you said you had to change the intake gasket yourself.

Going back to my first comment about AC DELCO.. was the fuel pump you put in it a ac delco fuel pump??

I did not do a tune up while fixing the gasket so the distributor, cap, rotor, plugs and wires are all the same that were there before. At the time that I worked on the gasket I didn't realize my misfire symptoms were for cylinder misfire, I had just assumed it went along with my leak (like I said, I don't really know anything about engines,). It was after fixing the gasket that I found out it's a separate issue when I turned the car on to make sure the leak was fixed and my father pointed out the rough idle and check engine light and then ran a computer test.

As to the fuel pump, I don't know personally as my father installed that for me last march.