RPM Drop From Nuetral to Gear

my buddy had the same problem with his 340 ..it woudn't idle down and would dies you put in gear .. he thought it was mechanical (maybe a timing chain jumped) so He switched the whole engine with a spare 360 and still had the problem.. well after a months time and two engines and lots of new parts installed ... It turned out to be the simplest thing.. It was a spark plug wire that had burned thru on the exh manifold. it had a dead cyl.
none the less .............he felt pretty stupid after ALL that .