1984 Other Toyota Models Hydraulic lifters 3y engine Toyota

I have had 2 "push rods" jump out from thier position in the rocker arm on 2 seperate occassions.

I changed the first hyd. Lifter and the problem moved to another push rod. I gave it to a machanic and he thought the oil was sludgy and he put a oil system cleaner through the system. And change the oil and filter.
The oil pressure seems fine but the rod still dislodges every couple of week. Is the answer to change all the lifter.
The rod pops out when I first crank the car over. The motor run fine for the couple of weeks then of course it runs on 3 cylinders.

I am trying to keep cost down as the car only has a couple of years left and it is rusting away from driving on the beach.

Please help as this has got me trick. Have you herd of this happing before?