Transmission fluid may be entering the intake manifold through vacuum modulator.
The Fix: Replace vacuum modulator
Cylinder head gasket(s) may be bad.
The Fix: Replace cylinder head gasket(s).
Cylinder head(s) may be warped or cracked.
The Fix: Resurface or replace cylinder heads. (Resurfacing is not a DIY job)
The engine block may be cracked.
The Fix: Replace engine block.