I swaped a b18c1 block and a b16a2 head into my 00 civic si
it has 11.5:1 cr
sometimes the idle seems to move up and down between 1000- 500 rmp.
this dont happen all the time, just ones in a while.
what can cause this problem
Im about to intall a p06 Ecu chipped for 310cc injectors and Gsr cams.
Car runs great but this is not a big problem yet

 

You may want to check out the IAC, make sure the the wire colors are correct (yellow/black and blue/black if I remember correctly) for the plug that is going to it. It's pretty common to see that and the IAT (green/white red/white) plugs get mixed up.