the car is an 89 740 ti....

i recently swaped the engine from my old 86 760ti into a much cleaner 89 740ti that had tossed a rod... engine ran very strong after inital swap... but now about 500 miles after flawless driving the idel is giving me some greif...

originaly i had just set the idel around 1000... prob a tad over... the other the car just suddenly started to idel around 700 rpm... that was nice as its where it should idel... but the prob is the idel would drop a little to low sometimes when coming to a stop... i tried adjusting the base idel with the TB knob with no luck...

note that the idel droping to the stalling point only happens after i've been in the car for about 10 min and gotten the engine fairly well warmed up.

along with this i have noticed a lot more "burping" from the exhaust (which is esentaly a glasspack in place of the cat and it exits before the wheel) much like when my mr2 would run rich... the other day it sounded like i got a nice backfire out of the system, likely spit some flames (thats what the mr2 sounded like when it spit fireballs)

does anyone have any idea what this could be? the only idea i have is the idel air valve... but wanted to get some other opinions before i started digging into the engine bay.

thanks
mark