[USR-TC] Reboots (watchdog timeouts)

We are getting a lot of card reboots. I get Management bus failure trap and watchdog timeout traps when reboot happens. On our one chassis, where NMC is V6.1.17, ARC V4.1.59, DSPs V3.1.7 we don't get any reboot, but on chassis where NMC is V7.1.8, ARC V5.2.102, DSPs V3.1.7 we get a lot of reboots (nearly 2 - 3 a day with 6 DSPs 1 ARC). What is reason for reboots? Is it NMC problem or DSP ? Does reflashing recycling power or something else help? Can I remove NMC card if I have static configuration of ARC (set chassis slot x type static ...) without disconnecting all users?