The stack consists of a 24TS and a 12S. It has about 40 vlans on it and 25 SVI's. Except for minor changes to access-lists, its config has stayed relatively the same for several years.
I added a 3750E to the stack over the summer and upgraded the firmware. A few weeks ago, when our residential students returned and traffic levels went up drastically, I started getting reports of poor performance, including packet loss of over 30% at times.
In all instances, it seems to be source IP dependent. Two clients on the same distribution switch, one with high loss, and one with no issues. Level 2 connectivity is solid.
I upgraded the firmware again and one of out mailservers now had issues. I removed the 3750E and this was fixed, but the overall issue is continuing.
I downgraded the firmware this morning with no change in operation. I do notice that clearing the ARP table seems to help, but it is only temporary. I've now had 12.2(35,37, and 40) all have the same issue. I have even replaced the stacking cables.
I'm at a loss now. I'm about to build another stack with spares and do a major swap, since I have run out of ideas.
(and the students are about to riot.)