The transmit lockups have been reported to occur in most cases on ports operating at speed 10 and duplex half.
The transmit (Tx) lockups can be recreated in the lab when there is a rapid increase in the rate of change of traffic transmitted by the Catalyst 2940. The Tx lockups are reproducible in the lab for all speed and duplex combinations but the Tx lockups are very difficult to reproduce on ports operating at speed 100 and duplex full.
This issue is discussed in Cisco bug ID CSCsb79517.
Upgrade the software image to Cisco IOS Software Release 12.1(22)EA7.
Move the device to a different port on the switch as a temporary workaround.
Software reload or power cycle the Catalyst 2940 to clear the transmit lockup.