Random cylinder misfire

[Chaztschida]

So ive had this issue for awhile. Months ago I blew the engine and just got a new (reman) long block put in about a month ago and it still keeps coming back. I've put new plugs in ( before the engine blew ) just put new plugs wires in and I cleaned the MAF the shop put a used coil pack on for free (one they just had and knew worked) they think just dirty injectors but additives have made no change.

Symptoms are just a rough idle no milage loss or noticable power loss. I can drive it like a mad man and no issue but if the rpms ever go over 5k the check engine light flashes and stays on. If I never let it go over 5k the light never comes on.

Any ideas before I have to take it back in to have it figured out? I'm at a loss