Try cleaning the wheels.

Hi Michael
Welcome to this great forum.
Difficult to provide you with a definitive answer but if the CCS800 stayed in its box since 1960 that you are probably dealing with corroded collector and frozen oil.
This means you probably need to open the loco, clean and oil it.
Here is the exploded view of it:

The central body can be removed by unscrewing 2 screws underneath.
To remove the side bodies is a little more complicated as you need to remove the lamp holders on each side before removing 2 side screw from underneath each cabinet.
If the loco has not been previously tampered with, then we need to check for Zinkpest (sadly it may happen on very old ones) and for corrosion on the motor's collector.
Once the bodies have been removed, remove the motor brushes and check if the motor is rotating freely. Don't try to rotate it through the wheels as there are worm gears.
With hear buds and a small brush dipped in lighter fluid clean gently the mechanism.
With another hear bud with most of cotton removed and dipped in lighter fluid, clean the inside of the brush holders.
Then add a little oil (Märklin oil or car motor oil) on axles and worm gears.
Then put back the brushes and connect the loco to a Märklin transformer and let the loco run for some time while checking nothing gets too hot. This will clean and restore the surfaces of the loco.

This is a wonderful loco, here is mine

Cheers
Jean

Sounds to me like something changes when the lok is turned uprite vertical. You state that it runs good on it's side. Perhaps something is binding when it's on the track. The old ccs has many moving parts. If it's overheating it sounds like it wants to run but something is holding it back. Be patient. Don't start ripping it apart.
Good luck
Mikey