Hey everyone,having some issues with my transmission and wondering where to start.

Some background... The TH400 was rebuilt approximately 8 years ago, has about 3000 miles on it. The engine is a 454 stroked out to a 496.

I have had no issues up until a couple days ago. The car was running rough (ie when I would step on it, it would kind of bog down ) then I lost 3rd gear. Not sure if these two issues could be related... perhaps a vacuum issue?

I did change the fuel filter, although not perfect the engine is running better... not bogging when I step into it.

Here is how the transmission now performs. I have first and second, (shifts normally manually and when in drive) and it will only shift into third when the rpm's get up there...at around 45mph. Makes no difference if I have it in drive or manually put it through the gears. Reverse works no problem. Fluid levels are good.

Vacuum issue? Kick down switch? Modulator?

Any thoughts on how to narrow down the issue?

Thanks in advance for any support.

Cheers, Geoff