please help

I have a 1995 Chevy Blazer 4WD Automatic

New engine has 40k

After a cold start engine idles very rough, rpms will be steady and then "nose dive " like it's being choked. Then it'll recover and about up seconds later do it again

When engine is warm that problem goes away but a slight shake when idling in Park or Drive persists throughout drive cycle.

Recently replaced spider injectors, plugs, wires,  cap, rotor, fuel filter, air filter

EGR valve looks very shiny and new, i think i had that replaced within last 10k miles

Shop says vehicle has P0300 , but they have no clue where to go from here, they mentioned that the timing is dead on, so they don't think distributor could be issue. Please help, thank you sir