Weird Idle 87 turbo II

[imthatasian]

Bought an 87 Rx7 turbo II last november... got it with the turbo disassembled, idk why it was like that... but either way it was cheap and i wanted to mess with it... this is my first import and first rotary, i'm usually a GM mechanic... so i have no clue whatsoever on this.... well anywho long story short.. i replaced the turbo with a used one which was in good condition, but when she first fires... she revs up to 4k rpms.... then about 3 min later she drops down to 1.4k rpms and fluctuates from 1.4k to 2.1k rpms.... i've replaced all vacuum hoses, it seems to me that it's sucking too much air... so i've messed with it for over 6 months, even brought it to the dealership and they couldn't even tell me what was wrong with it.... i was told by 3 people that it could be the thermal valve in the back that commonly goes out... she drives fine when she's under load... it just happens when she's at idle.. any ideas???