70 coronet original trans rebuilt about 500 hard miles ago-shifts good, doesnt slip at all but when I step on it the trans-shiter jumps into nuetral? I noticed it first a week ago after 2 hard runs through 1st 2nd and into drive, I got on the freeway leaned into it at about 50mph it grabbed for a second and jumped th shifter into nuetral-I tought I just ddnt hve thehiftr all he way into drive from before but it does it everytime I stand on it now, seems to drive fine when no goosin on it but I did very minimal driving because I didnt want to wreck anything else. Any Ideas would be helpful!

 

Check the motor mounts and trans mount and the linkage adjustment

 

yep busted motor mount been there done that.

 

My first thoughts also...

It's probably not the transmission.
It's the twisting of the drivetrain that is probably affecting the linkage....
Back in the day, we used to chain down the driver's side of the engine in order to keep from breaking the driver's side motor mount. I've never had a passenger side mount break but, have replaced transmission mounts more than once....