C6: help me fix 3rd gear buzz up

My granddad's 72 Mach 1, big block, c6 (trans brake) car has a chronic issue with not shifting into 3rd gear going down the track. 1st to 2nd is fine but when it gets to 3rd it falls on its face while the engine spins up about 5-600 rpm for a second before it shifts and then pulls fine. It's been an ongoing issue and we've had it rebuilt 3 x's in the last few years for this. Keeps coming back the same. Our trans guy says now that there is nothing they can do and c6 in in a heavy car behind an engine making any power will always do this, but we ran the car for 6 seasons with the trans working fine before this issue started, so I don't buy it.

What 2nd gear servo do you have?

find a new trans guy!!

I'm not sure what servo is in it.... I texted my dad to try and find out. Tell me more about the servo and what might be goin on, if you don't mind. Have you guys heard of similar issues others have had or does this sound like a unique situation?

P.S. We are open to finding another trans guy for sure. My dad started going to this place way back because at the time he knew of no one else that could make a brake work in a C6, but we haven't been pleased with results lately

Talk to Frank Merkl

Not uncommon. What valvebody? If it is TCI bin it or Frank has a fix

I know it's a billet valve body but I don't know who makes it.  The trans was put together by automatic transmission design in Wisconsin.  Based on everything I've read on this forum, I'm thinking we need to just get a hold of Mr. Merkl and figure out what we need from there

Franks  valvebody and set up his way is the go and know it works

 I'm about to build another C6 now exactly that. If Frank is busy or you need help just sing out

Can anyone get me contact info for Frank Merkl?

I think this will work..

https://www.429-460.com/u57contact