Falling PPM ...

So yeah, I have the opposite problem that most hydroponic growers have, My PPM nutrient solution drops instead of rising. I have perfectly healthy plants though. I know I need to raise the ppm slowly to find the sweet spot. At what intervals should I do this? 25,50, 100 ppm?

IMO that has to be done by what your plants are telling you. Raise it and see how they react, then decide how much to raise it again after.

Start with bumping it 50 and see what happens. Maybe bump it again next week or in a few days. You don't need to run high numbers, if the plant looks healthy just make some small adjustments.