We used to put dexron in for trucks run in cold storage. Straight hyd. would get too thick. Seems to me we would go up to about 30% dexron. This was per Clark factory advice at the time.

The shut-off valve sounds like a good way to isolate the problem.

Put a shut off valve between the main hydraulic supply and the mast. Raise the mast into second stage, shut off truck and valve. This will identify if it is hydraulic or cylinder. Should mast continue to lower it is in the cylinder. If it does not, check for voltage at the solenoid or through the dash display, diagnostics, hydraulics. Look at the lowering voltage. I suspect a phasing issue at the sequence valve block. It stops lowering once the hoist cylinders lower and does not continue, dig into the valve.

are the cylinders bypassing ?

This just came to me if it was a worn out sport inside the control valve then it would also do it on the first stage it works like a champ on the first stage. Would there just be more pressure when it's up on the second stage with three cylinders pushing down on it as opposed to one

Mmmmm I didn't think about checking the voltage thank you so much. Also if this helps out any I change the hydraulic fluid to a very thick viscosity because it had red fluid which seem like ATF which is very thin and that seem to fix the problem for about six months

Only other thing I could suggest is to monitor the voltage across the coil when it's malfunctioning to make sure it's not being told to lower. If you can determine that there is no diode suppression inside the coil, you might try switching polarity to the coil.

You may also have a spool valve that is not completely returning to stop or worn out allowing fluid to bypass loosing hold pressure.

No the forks continue lowering. There are two solenoids on top of the Lift lowering portion of the control valve I did swap those and the problem still is there

Do the rails continue lowering while the forks remain stationary? If so, the mast isn't staging properly. Innerslide lube and repeated lifting-lowering usually fixes this (in older, seldom used trucks).

If the forks continue lowering just as if the switch was still engaged, then you might have a sticking solenoid. On some old solenoids you could flip the coil and counteract the problem of a magnetized valve body. You could try swapping out the solenoid if you have a sister truck.