6850 Stack Acting up

[Hmm]

Hi everyone,
I have a stack of 6850s consisting of 1) 6850 copper, and 2) 6850 fiber switches. They are stacked with HDMI and have been working great for the past 3 years. Just last week we started seeing something strange with the stack. We lost communication with about 17 of the 24 IDF switches connected to the stack. The IDFs are 6250-24s, and the status on the ports in the stack said they were up but we couldn't communicate to the IDFs on any of the vlans. We confirmed by testing from user ports on the IDFs, and no traffic passed back to the stack. As a quick fix we tried rebooting the stack and after a few minutes everything came back up. Then a few days later it happened again and a simple reboot fixed it.

It was just about an hour ago the stack did it again. Only this time it took 3 reboots to restore connectivity. I don't understand what could be causing this issue and why the stack says everything is connected but traffic still doesn't pass.

We have checked the configs and nothing has been changed in over a year. The 17 ports that act up are the same with each outage. Also, all the IDFs and the MDFs are pretty much up to date on microcode.

Scratching my head here and I would really appreciate any input or advice on what else to check.

Thanks

[devnull]

6850 are not stacked by HDMI.. so that should be something different.
I never experienced anything like that.

Anything on console?
Anything in logs?
Any unusual traffic patterns interface util/cpu util? (fire up Wireshark, maybe a loop?)

Is it on the 6250 (IDFs?) or is the problem on the 6850?