Sherman wrote:

....
My 4 year old lawnmower gets hot and dies. ...
Rick says "Check the vent on the gas cap, too".

Well, Bingo! Rick wins.

After checking for spark after it died, and getting a positive
response from my wife on spark, I replaced the spark plug cap and
reached up untwisted the gas cap off a 1/4 turn and it started on the
first pull.

Now, I looked and I don't see any hole in the gas cap for it to vent.
I hate that she has to mow with the gas cap not tight. She might
catch the mower on fire. I'd hate to lose that old mower.

Do I just drill a small hole in the cap?

It's a four-year old mower and now a non-vented gas tank is the
problem? Don't think so...

Mayhaps the vent is plugged, but I'm still not convinced...there's got
to be an existing vent if that is the problem or it wouldn't ever have
run reliably...