maff help

[adam76]

HELP last month i had a problem with a ruff idle and hunting engine on tick over, and would bed down at low revs in all gears did code check and threw up maff so changed maff and did emcu reset all was going well until today started playing up again with same problems under advise from a friend popped the old maff back in and bugger me all went ok !!!!!! ??????? so my question is why would this happen and is there a cure mine is a 07 n/a hawkeye tia