1990 NIssan 240SX engine abnormal

[Racerfanny]

I have recently installed a semi-new engine into my 1990 nissan 240sx. And its giving me a problem. First, during start ups, its takes a while for it to start and once it starts slowly, its takes a while for it to reach its idle zone, about 7 seconds. Also, when i snap the throttle, the engine whats to die (and sometimes does die). But when it doesn't die, It shakes a Little, revs high and low repeating until it finds its idle zone. Its doesn't do this anymore but it use to misfire during cold start ups. But the funny thing is, that one i start driving it around(Freeway speeds, not stop and go traffic) its completely normal. Any idea of what could be wrong?

FYI: The engine doesn't knock or have any weird noises. its just acts weird.

[brndck]

start with the basics.
have you done a compression/leakdown test
have you had the injectors flow tested
have you set the timing
have you set the gap on the spark plugs

[slow40sx]

all that stuff ^^