You can diagnose or guess.

To diagnose, get a 100 # gauge and attach to the governor port.

At start up standing still, you should have 0 #’s of pressure to max of about 1. Start to roll away from a stop and the pressure should mimic vehicle speed.

Come to a stop and the pressure should fall to 0-1 #.

If higher you have a stuck/sticky governor.
If 0-1 and it launches in second or third you have a sticky shift valve or governor plug in the valve body.