Transmission Sensor ?
 

I have a 2002 Elantra Auto...

I had a problem a year ago where i'd be driving at highway speed and the
transmission would drop to 3rd (fail safe) and would only return back
normal if i pulled over and switched off.. This turned out to be the pulse
generator.

A new problem is now effecting me... From switching on i drive from 1st to
2nd then high revs before jumping to 3rd.. Then i remain in 3rd when
speeding up or slowing down.

I switch off.. back on and it drives like normal.. I switch off back on
and the same error occurs. I've not determined if this is from a cold
start only but it's at the shop now so we'll see.

Any ideas what the problem could be?