I can't figure out if my problem is mechanical or electrical...

For a while now (about 5 months) when I've been driving, sometimes my
speedometer will drop from my speed (say 55mph) and fall to 0mph, then
spring back and forth between 0mph and 130mph. Several months ago it only
happened on the highway when I was going 75mph (about) and now it is
happening at all speeds. Within the past 3 months whenever that happens with
the speedometer, my transmission (automatic) will disengage and sometimes
shift itself into 3rd or 2nd gear while I'm going like 70mph. Eventually it goes
back (quickly) and I continue driving but recently (past month) when it happens
the transmission becomes disengaged and the motor will rev and the rpms will
go up but there isn't any acceleration. I've had to pull over on the highway,
shut off the car, wait, restart it, and then it drives absolutely fine. To me, it
seems it only happens on days where it is very hot outside and because I don't
have AC and I have to park it in direct sunlight, I think the heat might be
affecting something electrical (it's been about 100˚F-ish these past couple
weeks) and it has been acting up more. I've already replaced the module (the
one under the floor on the passengers side), but I'm not sure if there is
something that I may need to replace or just something I should look at. I used
to think the transmission was starting to go up, but it's been going on for a
while now and I didn't think it would be a mechanical issue because it wouldn't
have gotten better, or kept up for this long (I figured it would have died by now
if it was a transmission issue). It's a great car and it's been in the family since
1992 and I'd like to keep driving it.

Any suggestions on what the problem may be? Or ever heard of this before?