This past weekend I connected a 3750X-48 2-stack to a 3750X-48P using fiber GBICs. An interesting thing happened when I did the no shut command on the first interface the channel group comes up fine. When I no shut the second interface I lose connection to the stack for 6 seconds (I know this from the number of pings I lose) and things are fine. If I remove either inetrface from the channel group either physcially or logically the same lose of connectivity happens to the stack. I am using a config template from about 30 other stack connections, verified that STP is correct (or at least the configs match) and swapped cables and GBICs.
Looking for any insight on this or other things to try.
When I have the port channel up (2 connections bewtween the switches) on other switches and pull fiber cables to test connectivity I do not lose a single ping. Management VLAN1 stays connected with no issue and I do not lose traffic to devices on the far switch. Thought it was odd that with these particular switches they do not act as the others.