engine swap now low idle/ dies out

so i did my engine swap and now my idle will some times be very low (like right above 0) or even it might just die out then at othere times it will be to high of an idle. i dont know if this would/ could cause any of this but my exhust pipe broke off right before the cat.
any one have any ideas of what is causing this?

thanks
mike

If there was a rather large vaccum leak i'm sure that could cause it to die.
Did the new engine have the intake, throttle body and sensors on it already?
The reason I ask is maybe there is some problems with the sensors on it.

i put the intake and every thing from my old engine to my new one

I had this problem on my 87 & replaced the tps & the Idle contol solenoid, o2 sensor,chased phantom vacuum leaks all in vain,then after 2 years of stalling/high idle problems it wouldn't run-nospark,so I installed a reman distributor still, no spark so i replaced the coil & ignition module & not only did it start & run fine it fixed the idle/stalling so if you have another coil & module try it, It worked for me and the wasn't even the reason I changed the part. evidently those things can cause intermittent problems for a long time before they completely die.