or maybe its a vacuum leak you cant see - and an exhaust leak thats bad could do a similar thing - do you have proper vacuum when its running? - 15inHg minimum i mean.
What about a boost leak test? Have you done that?
also - y doesnt your tach work? It's kinda something you want on a car you modify
also - what else is modified and what else did you change besides hte turbo?
beleive it or not - i had a similar weird idle that went away when i did a rad flush at work (pressure test/fil) you could try burping it (i assume you had to pull coolant to do the turbo swap - or you made a huge mess) So - first id list whats done here - then i'd check my coolant and burp the system, then boost leak test/ vac test, and then test the iac and cts voltages.
Since it didnt do it b4 and it does it now it's likely to be something that you touched (not so likely its a iac or cts) did you change the bov?