1999 Mercury Villager engine staling

This car has been to a mechanic several times for the same issue.

A rattle from underneath the front seats, then engine misfire, engine sputter ( jerky sensation ) with eventual stalling.

On first visit to mech.
The distributor was replaced with used and a full tune up on first visit. Problme seemed to go away for a few weeks. Then same problems arose.

Second visit

Fuel filter changed and lines cleaned, and distributor replaced with another used. Problem seemed to go away and came back.

Third visit replaced fuel pump. Problem seemed to go away and came back.

There is no engine check light that comes on. The computer does not display any errors.

The mechanic seems to be stumped. He wants to replace distributor with new ( 500.00 more ) over the 2,500.00 to date.

Any suggestions for this problem. Could it be the exhaust. Car runs for 20 to 25 mins before showing signs. Leave car alone for a few hours and it restarts immediately with no signs until it warms up.

Thanks

TR

tlr280@optonline net