Ok, I have had several issues with my TH350 Trans. First had a leak in my trans cooler lines and replaced them. Filled the fluid back up and drove the car to the gas station, The car wouldn't shift out of 1st gear and the shifter would not shift down to second and Low gear. As much as I stepped on the accelerator the trans wouldn't shift but the RPMs would go up. And there was no torque. The would be better torque if I let up on the gas and let the RPMs drop. I took it to the trans shop but they only gave me a little time and they checked the shift modulator and it was functioning properly as far as vacuum. They did find the rubber tubing which provided the vacuum was going bad so they did replace that. I flat bedded the car home and replaced the shifter cable and now have full shift range. The transmission shop said I could have a bad governor. since I had the car up on cribbing, I purchased a NOS governor. I got the existing governor out this evening but it looks and seems to function properly. No knurled gear, valve moves freely when the weights are opened. I'm hoping to get the governor back in tomorrow, if I can find a parts shop that has the correct "O" ring ! Three parts stores today and no one carries one !!!!!!

 

More info:
History?
How many miles on it?
Ever been rebuilt? Service record?
Is fluid level ok?
Have you dropped the pan to inspect for the amount of debris?
Is the modulator line free of obstruction?
Is the modulator line hooked to a direct intake vacuum?
The next step is to verify the line pressures. Get a good 300psi trans gauge and read the pressures in all 6 ranges.
You can pull the gov and inspect the gear and the shaft bore but there are several things mentioned that come before that step...