Here's an odd one for you:
I have a 2821 with a WIC-1CSU-T1 that's been running 12.4.2T2 for over a year with no problems. I tried to upgrade it to 12.4.6T5 and as soon as traffic starts to pass through it I get MAJOR errors and overruns. If I roll back to 12.4.2T2 everything goes back to normal.
I've replaced the WIC, but the same issue persists. I have an out-of-band modem attached to the AUX port, and have tried changing the serial 0/0/0 interface from frame-relay to HDLC, put it in loopback mode, and ping'd the heck out it (using various packet sizes and ping patterns (all one's, all zero's, etc) and NO packets were dropped and no errors appeared on either the interface, controller, or service-module.
I've even tried going up to 12.4.9T2, same exact issue. I even have ANOTHER 2821 at a completely different location with the same problem. However I have 80 other 2821's that upgraded fine with no issue, and they all have nearly indentical configs.