On engine start up, look, listen for broken vacuum lines. They are fragile and most all crack eventually. They run across the firewall colored black, red, brown. If big enough leak, then spraying a mist (aerosol carb cleaner, Seafoam spray, ect..)near the crack will show itself.
A full PM is always helpful at eliminating any guesses.
-Clean air filter, MAF sensor, clean intake. (Seafoam treatment through vacuum lines once a year helps)
I even added a home made oil filter/separator inline with PCV system.
-New fuel filter. (once a year for me)
-Could be a leak that is effected by expansion/contraction of the engine.
-Seafoam is great for cleaning intake. Techron is the best made for adding to gas tank. Perform treatments shortly before oil changes.
'97 3.8L V6
1996 SVT Cobra White