older v6's had hydraulic lifters that have small oil passages and if they get sticky one might flatten every now and then especially on cold mornings. I think i've known 3 trucks that did this, the bigger lifters came in '98 i think. For the price if the truck is otherwise in good condition buy it and put a motor in it or do a rebuild.

1999 Montero SAS'd on Kings and stuff

1998 Montero trying to get a V8 Swapped