Odd problem. Input seems robust on the paddles, with zener and cap. I'll echo what Steve asks, does this happen with a dummy load? Is it all bands? I wonder, if you put it to the lowest CW speed (on keyer), if a dah will lock it up also (might not, might not be long enough). Just to verify, nothing in the accessory jack? Just to rule out another path for RF to get into the radio.