When going up a hill between 25-30 mph my automatic trans starts surging between 2nd and 3rd gear. Every other speed is fine on flat roadds and up hills with no other surging. However, my blazer does clunk occasionally when taking off from a fulll stop. Please advise.

 

What transmission do you have, TH350 or TH400? Is the vacuum line connected to the vacuum modulator and is this connected to a ported vacuum source or a manifold vacuum source?

 

Unfortunately I am not able to slide underneath the vehicle, but I do believe it is a TH400. I have no idea about the other questions you asked.

 

They both use similar modulators. I would recommend a replacement, adjustable modulator (typically called the red stripe modulator since it has a red line painted on it).

The vacuum source for the modulator needs to go to the intake manifold. Some of the ports on the carb are called ported ports which means that they may not have vacuum at idle or low throttle angles. A ported source is not correct for the transmission modulator valve.