The R38 potentiometer it's has to be replaced.
Edit: the potentiometer was replaced (and also a "hack" on the motherboard to enable the S-Video signal), here is the sample:
Based on the information provided, I'd recommend the following first steps:
Clean the monitor jack on the 800XL using contact cleaner.
Inspect the monitor jack while inserting and removing the cable several times. It should slide in and out smoothly and the pins remain clean. Also the monitor jack itself should not flex or move appreciably.
If there is any ...