Random reboot of 3com 4400 stack

Can you help, please? I have a 3Com 4400 stack running the latest version v6.13S. The stack sometimes reboots randomly. The logs showed the following: Unit 1 Watchdog handler is RESET The History Log Id No S/W Rel Reason UpTime SysErr 42 42 6.13s S/W Watchdog (r) 17 Hrs 42 Mins 52 Secs Uptime Hours 16828 Reset 41 43 43 6.13s S/W Watchdog (r) 0 Hrs 51 Mins 14 Secs Uptime Hours 16828 Reset 42 44 44 6.13s S/W Watchdog (r) 0 Hrs 47 Mins 54 Secs Uptime Hours 16828 Reset 43 45 45 6.13s S/W Watchdog (r) 0 Hrs 32 Mins 15 Secs Uptime Hours 16828 Reset 44 46 46 6.13s S/W Watchdog (r) 0 Hrs 18 Mins 48 Secs Uptime Hours 16828 Reset 45 47 47 6.13s S/W Watchdog (r) 0 Hrs 33 Mins 59 Secs Uptime Hours 16828 Reset 46 48 48 6.13s S/W Watchdog (r) 1 Hrs 9 Mins 57 Secs Uptime Hours 16829 Reset 47 49 49 6.13s S/W Watchdog (r) 0 Hrs 59 Mins 28 Secs Uptime Hours 16829 Reset 48 50 50 6.13s S/W Watchdog (r) 0 Hrs 42 Mins 37 Secs Uptime Hours 16829 Reset 49 51 51 6.13s S/W Watchdog (r) 20 Hrs 17 Mins 57 Secs Uptime Hours 16849 Reset 50 52 52 6.13s S/W Watchdog (r) 0 Hrs 5 Mins 19 Secs Uptime Hours 16849 Reset 51 53 53 6.13s S/W Watchdog (r) 0 Hrs 55 Mins 15 Secs Uptime Hours 16849 Reset 52 54 54 6.13s S/W Watchdog (r) 0 Hrs 37 Mins 23 Secs Uptime Hours 16849 Reset 53 55 55 6.13s S/W Watchdog (r) 1 Hrs 26 Mins 17 Secs Uptime Hours 16850 Reset 54 56 56 6.13s S/W Watchdog (r) 1 Hrs 42 Mins 12 Secs Uptime Hours 16851 Reset 55 57 57 6.13s S/W Watchdog (r) 17 Hrs 26 Mins 34 Secs Uptime Hours 16868 Reset 56 Assertions : Id S/W Rel File Name Line 5 3.00 agg_sma.c 1215 6 3.00 agg_sma.c 1215 7 3.00 agg_sma.c 1215 8 3.00 agg_sma.c 1215 9 3.00 agg_sma.c 1215 10 3.00 agg_sma.c 1215 14 3.00 agg_sma.c 1215 Last System Error Data: ID (57) S/W Rel (6.13s) Reason (S/W Watchdog) Unit 2 Watchdog handler is RESET The History Log Id No S/W Rel Reason UpTime SysErr 236 236 6.13s S/W Watchdog (r) 0 Hrs 10 Mins 55 Secs Uptime Hours 48145 Reset 235 237 237 6.13s S/W Watchdog (r) 2 Hrs 2 Mins 20 Secs Uptime Hours 48147 Reset 236 238 238 6.13s S/W Watchdog (r) 0 Hrs 29 Mins 15 Secs Uptime Hours 48147 Reset 237 239 239 6.13s Power (r) 2 Hrs 0 Mins 0 Secs Uptime Hours 48149 Reset 238 240 240 6.13s S/W Watchdog (r) 17 Hrs 5 Mins 9 Secs Uptime Hours 48166 Reset 239 241 241 6.13s S/W Watchdog (r) 2 Hrs 55 Mins 1 Secs Uptime Hours 48168 Reset 240 242 242 6.13s Power (r) 3 Hrs 0 Mins 0 Secs Uptime Hours 48171 Reset 241 243 243 6.13s S/W Watchdog (r) 91 Hrs 47 Mins 28 Secs Uptime Hours 48262 Reset 242 244 244 6.13s S/W Watchdog (r) 45 Hrs 25 Mins 25 Secs Uptime Hours 48307 Reset 243 245 245 6.13s S/W Watchdog (r) 4 Hrs 29 Mins 38 Secs Uptime Hours 48311 Reset 244 246 246 6.13s S/W Watchdog (r) 117 Hrs 3 Mins 10 Secs Uptime Hours 48428 Reset 245 247 247 6.13s S/W Watchdog (r) 0 Hrs 32 Mins 9 Secs Uptime Hours 48428 Reset 246 248 248 6.13s S/W Watchdog (r) 3 Hrs 11 Mins 54 Secs Uptime Hours 48431 Reset 247 249 249 6.13s S/W Watchdog (r) 20 Hrs 52 Mins 50 Secs Uptime Hours 48451 Reset 248 250 250 6.13s S/W Watchdog (r) 3 Hrs 5 Mins 37 Secs Uptime Hours 48454 Reset 249 251 251 6.13s S/W Watchdog (r) 1 Hrs 42 Mins 13 Secs Uptime Hours 48455 Reset 250 Assertions : Id S/W Rel File Name Line 129 6.10s lazy_ack.c 890 137 6.10s poller.cc 1104 138 6.10s poller.cc 1104 152 6.10s lazy_ack.c 890 158 6.10s lazy_ack.c 890 159 6.10s lazy_ack.c 890 169 6.10s lazy_ack.c 890 170 6.10s lazy_ack.c 890 173 6.10s lazy_ack.c 890 175 6.10s lazy_ack.c 890 187 6.10s lazy_ack.c 890 191 6.10s lazy_ack.c 890 199 6.10s lazy_ack.c 890 208 6.13s lazy_ack.c 890 209 6.13s lazy_ack.c 890 225 6.13s lazy_ack.c 890 Last System Error Data: ID (251) S/W Rel (6.13s) Reason (S/W Watchdog)