$0D TBI build - spark problems

[dzidaV8]

Hello, I'm having some issues tuning my fresh built engine.
Engine specs:
1998 L31 Vortec bored to 355cid, flat top pistons,
LT4 HOT camshaft,
GMPP TBI intake, 61lbs injectors (for now).

I'm using '427 ECM, xdf and adx from this site (attached). The problem is that commanded spark advance is about 10* lower than values in Main Spark Tables. When I set main spark bias to 0, the values start to match. XDF has bias value in conversion formula. How is this possible? I zeroed all the other spark tables I could find, and set all spark multipliers to 1, still the same.

Another strange thing happening is that when I zero the main spark bias and have proper spark timing, the engine will idle for a while with spark values that match the main table, then it starts to retard timing to negative values. When i rev it a little it will go back to normal values and after a while retards again. I can't find anything that would do this in my bin.

I'm using the attached modified BJYL bin with stock 98 vortec spark tables.

Thanks for any help!

EDIT: One more thing - my IAC is not working and it throws IAC fault code 35 and MAP high code 33. MAP is good, only it idles at 70-80kPa when the timing gets retarded and it triggers the code.