88 iroc-z 350 tpi t5 swap

Got this car, it has been sitting for a long time drove it for about a 2 weeks no problems. Then out of the blue it seems like it misses and chuggs. It seems like it has a loss in power,yet it idels fine and will rev to 4500 ok but its real choppy.

Normaly ill drive it in the morning and it runs great then ill park it 30 min or longer. Ill get in it and notice its running bad and the longer you drive it the worst it gets.

i dont know what to do its got new wires, plugs,and icv also hot wired electric fans to switch

any ideas on what to look for would be great thanks...