Well, I've got a 98 RST. T3/T4, walbro 255, vortech SFMU, stock injectors and coil. Haven't had any problems with it (engine wise) since I got the new motor in until recently. As soon as I crank it the rpms' drop down to around 550-700 and it sounds like it has a massive misfire and it will stay like that until I run the rpms up to about 2500-3500, hold it there for a second, let off the throttle and it will idle normal and run fine the rest of the time until it cools down again. Vacuum drops down alot until I rev it up and goes to normal after I do hit the throttle.

It does it only during warm up, every warm up. I'm going to swap injectors when I get a chance to the 330s I have just hanging out in my room and see if that's the problem. But any other ideas are more than welcome.

I always like to start with the IAC when it comes to idle issues with our motor. Take it off, clean it thoroughly, OHM it out, and if all is well reinstall it.

I had a similar problem a while back, mine was the IAC. If you are thinking the injectors are too big for idle, what does your afr look like when idling? But like ^^ said, test the IAC and clean it before you go swapping injectors.