I found the bug today. It wasn't the software - it is hardware related.
I have connected pin 2 of the midi connector to the PIC's MCLR. For some reason, when several midi messages arrive rapidly, the processor reboots. I assume it's induced noise on the line as I don't think the little phatty outputs anything on pin 2 (and it doesn't happen when just one or two messages arrive).
Fortunately, I foresaw that it could be a problem and added a solder blob jumper. After removing this the mpg-200 works as it should (but it cannot be programmed of course).
I have added a temporary normal jumper, so now things work again. I can just remove this jumper before shipping.
Btw: the midi connector didn't fit so i had to remove some of the copper and drill a new hole for the shield connector. Rookie mistake...