I hate VATS!
Ok here's the deal, The little pellet on the key is nothing more than a presicion resistor. There are two little fingers internal of the key switch that sence the resistor. If the fingers are weak or corroded the resistor value gets higher than spec and the VATS thinks the owner is stealing the car.
( Nice feature huh?) ( or if the key itself in gunky)
Now you stated that you put in a new key switch & a new key didnt help.
Which leaves the following question, Did the dealer
replace Just the switch ( using the existing wiring by soldering on the new switch) or replace the whole switch & wiring down to under the dash?
A quick Fix ( for the brave at heart)
Remove the cover panel under the dash
find the 2 pin connector ( Small white wires, black jacket, brown connector right side of steering collum)
Get a buddy with a GOOD ohm meter, measure the resistor value on the key itself
Go to radio shack, buy matching resistor
remove connector under dash and solder the resistor across the wires that are NOT on the steering collum side but VATS side
Leave connector unplugged
Start car... see if that fixes your intermittant problem.
If it does your wiring up to the switch is bad.
If it dosen't your VATS module has a corroded connector OR is going bad.
Hope this helps
Vigman