Howdy Kirk.....Hope this doesn't sound too obvious but have you tried
any rudimentary troubleshooting yet? Though I don't know your
particular keyer, I'd suggest opening it up and either running it or
letting it stay powered up while testing to see if it works the same or
is now ok or better (air circulation to confirm heat related)....during
this time observe components (touch or temp probe with your meter etc)
for excessive or isolated warming (one component or one area etc). Once
the problem occurs, if warming (a ballpark would be parts getting
uncomfortable to touch) is observed, try hitting them with a jet of
freeze spray (or even cool compressed air) to see if the problem
changes. Check basic power supply voltages (look for a regulator chip to
get the value and/or check the basic specs of various chips or the one
keyer chip for its intended operating voltage). One specific thing that
comes to mind is that some keyer circuits utilize diode logic switching
at the paddles (diodes might be elsewhere though) and in particular
sometimes a diode is across the paddles......it "might" be that if this
was failing you could get the symptom you described. This basic
troubleshooting can be done without schematics if a little care and
common ham-knowledge sense is used so..........have fun.......let us
know what ya found.

73 - Gord - VE1AJF