Ph instability in early veg, what am I doing wrong?

I'm a lurker. Finally made an account looking for help.

Info: First grow, 600w MH, 2'x4' tent, ebb&flow with hydroton. Feeding with General Hydroponics 3 part stuff.

I've been having problems maintaining the ph of my system. Ph drops, ppm rises. I have not been adding more food to the reservoir. I've switched out the water and flushed the system a couple times, but that only seems to stabilize for a few days.

I don't know if its related, but a couple of my plants are getting brown tips on their first set of leaves. Any ideas?
My entire grow journal, with pictures, is available at http://ness2u.wordpress.com. Or, below is a picture of the shit on the leaves.

Well I wrote out a manifesto and deleted it all after I actually clicked your link and looked at your journal. 950ppm is WAY too much for plants that size. You are cooking them. Water is evaporating after your flood cycle and since the plants are too small to eat that large amount the ppm in your res is rising and the ph will drop. I would just begin inching up to 400ppm with plants that size using the GH 3 part. This is a simple a diagnosis as it gets. Back of your ppms bigtime.

I would suggest flushing out your hydroton with some plain water for a bit then start back with 350ppm. Please don't follow the directions on the bottle. Hope this helps.

Bucees hit the nail on the head. Water is evaporating and making your nutrient solution more concentrated and causing your ph fluctuations. It looks more like a ph issue then over feeding to me, although I agree 950ppm is a little strong for them younguns, especially if the ppm keeps raising on you.

Thank you. I will flush the system and give it a shot at 350-400ppm.
The water in the reservoir is only a week old, should it be getting cloudy? Is it a symptom of over-feeding?

Cloudy and smelly res water is usually the first indication of biological activity. This usually begins when the temperature of the res is over 68 degrees or light is entering your res. Algae will begin to bloom in there with a little light and cyanobacteria will grow in higher temps. The Cyanobacteria will morph into full blown Pythium/root rot if not handled correctly. Remember reading in those tutorials about keeping your res at 68 degrees or below and lightproofing it? This is why.

My res is ~61F. I'll take a look at how I can increase light-proofing or test for bacteria.
Thanks again for your help.