I'd suggest checking all your connections by recording anything while using headphones thru the jack. Wiggle all the connections and listen for pops and dropouts. I myself would doubt the M1, I've never had problems like that with my d100 (essentially the same deck as the m1) in more than 6 years with that unit. However, I had a lot of drop out problems with 1) bad mic cap wiring 2) bad battery box wiring and 3) bad input plug wiring.
my 2 cents, good luck! and please let us know what you find out