1995 SL1 auto transmission

Just recently in hot weather and stop-and-go traffic my '95s transmission
with 137,000 miles started to skip 3rd gear.  It would rev high in second
and then finally shift into fourth like nothing was the matter.  I stopped
for a while (to cool it off) and then it shifted properly.  Soon after I got
up to 55mph again it slipped out of fourth gear and I was pulling about
6500rpm.  I quickly pulled over and kept my speed below 30 (good for second
gear under the circumstances I thought).  When I couldn't avoid the highway
anymore (after a couple miles of 25-30mph) I entered the highway (55mph)
and it shifted like nothing was wrong.

I know that the car doesn't shift into fourth gear during the winter until a
proper temperature is reached, thus being in third gear for extended periods
of time here in MN.  I was wondering if this could have a wearing effect on
"third gear"?  Most of my driving is at night out of heavy traffic (65mph)
and this has not happened then.  When I do drive during the heat and heavy
traffic though is when I am concerned something serious could happen.

I have also experienced the "reverse slam".

Any insights will be greatly appreciated because I know a new transmission
will probably cost more than the car lists for.

Thanks in advance,

Pat