<VV> unusual symptom non corvair

[Michael Kovacs]

1987 Dodge van 360 engine. Occasionally while driving (twice) the lifters start to clatter and the oil pressure indicator drops very low.  Stop and check oil quantity (full). Upon starting, the oil press returns to normal and lifter clatter goes away. Fresh oil and filter within 1,000 miles. The oil pump is VERY difficult to get to ($$$$). I cant believe it is bad.
 
 Anybody have a clue? Suggestions are: 
clogged or partially clogged oil screen, 
pressure valve sticking open.
????
 
 I may try an ATF flush and new filter and oil.
 
MIKE KOVACS