nightfear

I have a MZR900 and i record live concerts. The problem i face for the last 2 months is that in some concerts the recording is stopped by itself. I have the HOLD key in ON position the SYNCHRO in OFF; no stick connected (the one with the earphones). I record with LP2 in a 80 SONY premium MD (always new). The battary is fully charged and i have a alkaline AA at the pack for extra record time. And i don't know why in NO specific time the recording stops. I took it to SONY service but they didn't find anything wrong ... Please if anyone faced the same problem please help !!! THanX .