i have a 1989 gmc r3500 and i believe the th400 transmission is going out.it will get up to about 40 just fine but when it needs to go into 3rd gear it like it was shifted into neutral and just revs..if you let off the gas completely and then press it again it will go into 3rd gear for a few seconds but will just rev up and not accelerate again after about a few seconds.i was wondering is it possible for the torque converter to be bad and cause this or am i more then likely going to have to rebuild it.also i just put in a new radiator that has a trans cooler if that can cause any problems.

You lost third clutch. Rebuild.

is it something that that you could do yourself or should i take it to a shop?

Take it to a shop, unless you are an experienced transmission rebuilder.

I could do it. You may be able to. I don't know. Check out YouTube on rebuilding them. They have a ton of stuff.
Here'* one.