Recently my 66's automatic tranny has had trouble shifting. It wont shift out of 1st until 40+ mph so I have to use the selection for starting in 2nd gear. Even then it wont upshift to 3rduntil 50+ mph. The fluid is clean and at the correct level and no mods have been made to the car in the last 10,000 miles except discs up front 2,000 milesago.Any suggestions for trying to diagnose or fix the problem?

Also, what is the proper way to change the coolant in the car? Is it as simple as draining and refilling or should the system be flushed somehow? Any tricks to remember?

Thanks for the help