I really gotta take some pix of the timing drive pulley-
on ALL of the 3l's I've taken apart, the timing pulley has SUNK onto the crank by at least a few thousandths.
Plenty to let the bolt loosen up!
Thus, I'd add 'timing pulley recession' to the list of possible causes for loosening.

fwiw,
t


'89 V6 4dr- '4- Doris'