In the end we unstacked the switches, ran a firmware update to no avail, then noticed the console port was working since unit 2 was no lo9nger stacked, and we could see the process causing the restart.  It was different every time, but only happened under load so we figured it was a RAM issue.  Switch replaced under warranty and all ok now.
We have two 7024 running image 5.1.3.7 connected with a dual CX4 module in each switch and two cables crossed to provide the stack links.  All looks good, but in four days, the standby (unit 2) has left the stack and come back again 4 times. I notice there is a new firmware available within the ;last few weeks, but it does not mention this particular issue as fixed.  Anyone had the same? I haven't had chance yet to re-seat the modules/cables which I understand are hot pluggable, so will try and do this tomorrow.  Below is syslogged events but I can't tell what the standby switch was doing or detected when it went wrong.  Any ideas how to track this or do I need to plug a cable in and debug/capture it via serial port?   TRAPMGR[318409280]: traputil.c(637) 4779 %% Stack Port Link Down: Index: 100 Unit: 1 Tag: 1/1 TRAPMGR[316653392]: traputil.c(637) 4780 %% Unit 1 identified a link failure in the stack. 1/1 TRAPMGR[318409280]: traputil.c(637) 4781 %% Stack Port Link Down: Index: 101 Unit: 1 Tag: 1/2 TRAPMGR[316653392]: traputil.c(637) 4782 %% Unit 1 identified a link failure in the stack. 1/2 UNITMGR[299082880]: unitmgr.c(6918) 4783 %% Admin configured standby 2 left the stack UNITMGR[299082880]: unitmgr.c(6934) 4784 %% No Potential unit to configure as Standby when unit 2 left TRAPMGR[316653392]: traputil.c(637) 4786 %% Unit 2 is removed from the stack
7024 stack member restarts