I think my transmission is going but I would like to hear from an expert as to what the real signs are.

1st seems fine, 2nd seems ok but 3rd and 4th will flare all the way to red line and sometimes don't catch at all but other times seem fine and the car be driven for 100 miles (or more) at highway speeds.

I first noticed the problem when I was driving at 65 or so in overdrive and would let off the accelerator the rpm's would drop down to 500-600. I have not changed the fluid because the mech said "if it's about to fail, changing the fluid will be the last straw. So don't change the fluid unless you are ready to shell out the $2700 for a rebuild"

Any help will be appreciated.