My shift linkage is well sync , no grinding at all and smooth shifting. All my bushings are new, shift rod cone screws are all tight in there .

Here is the issue.
If I am driving the car a bit aggressive, sometimes I would loose 4th and 5th, if I quickly move the shifter in neutral left and right (jiggle)a few times, they come back.

My guest is clutch cable adjustment or shifter itself.
Any ideas ?

Check your motor mounts.
If aggressive driving makes it do it, your engine could be moving around and changing the shift rod alignment.

If not those, verify your coupler at the fire wall is in good shape, too. Inspect it closely as it will look ok and will crumble easily when messed with when it is old.

All mounts are new. Motor mount is the six conversion from Rich Johnson, installed about six months ago and about 1000 miles.

Firewall bushing is the tangerine ( I think bronze ) that rivets to the firewall and was install the same time as the engine mount.

Transmission mount is new as well.

I will double check to make sure there is nothing loose.

Thanks all

Does the trouble happen when you're going straight or when in a turn. .
If only when turning hard, then maybe body flex. .

My experience is bad clutch adjust is usually mostly prevalent when going into 1st at a standstill. .

You may want to try changing the shift shaft joint one spline over in the tunnel at thre joint and see if that makes an improvement. . . and the 1st / rev plane still works OK

Could be the rod coupling below the shifter. Perhaps not tight enough and jumping a spline.

^^^ That ...


If it only happens in corners, your body is flexing to the point where the shift rod gets in a bind. BTDT.

Stick your head in the engine bay, rev the motor quickly up and down to see what it does, how much torque makes it move, and what moves....

It happened when going straight as well, not just comers . I will check the shift rod for spline jumping. .

Paul
Have an assistant take the driving position.
Chock the wheels or set the ebrake.
Then have the driver select first and slowly tease the clutch while you watch the motion of the engine. Do the same in reverse.
If the engine is lifting excessively you have an issue with the mounts causing the shift linkage issue.

Problem found ,
When I modified the shift linkage, I was a novice at welding, i still am. I inserted a smaller dimension metal pipe inside the factory linkage pipe (rod) first for better support then weld. The weld came loose and the only thing that was holding the shift bar was that extra pipe. If I didn't insert that pipe the linkage would be on the floor.

I took it to a professional welder today and the problem is solved.