Popping Out of 5th Gear (sideshifter), really don't want to rebuild the trans...need ideas Options

[rcrgyrl]

Working through issues with this new to me car. Took it for a great drive today.

But...I'm having issues with 5th gear.

Symptoms -
Popping out of 5th Gear when I lift, especially during downhill stretches when you don't need the throttle. Generally won't pop out when I'm on the throttle.
Harder to get into gear when going down hill as well.
Usually can get it back into 5th when it pops out if I match the rpms
Can sometimes hold it in 5th but not always and I'm afraid trying to do so is doing awful things to the inside of the tranny
With car in the air, when I put the shifter in 5th, I can push on the linkage at the trans and feel it pop into gear (I think)
Reverse and 1-4 all work nicely

What I have done -
Replaced the firewall bushing and both bushings at the end of the shift rod linkage at the tranny.
Inspected shift lever and linkage and rod and see no points of failure

I'm hesitant to try and adjust the linkage because the other gears are all working quite well.

Any thoughts, or am I just going to have to pull the tranny and have it rebuilt?

Thanks in advance!

Rcrgrl
'75 1.8 Dual Carbs