Curious - did you rinse the hydroton rocks good before you used them? If not, you'll still be okay, but I personally think it causes the behavior you are seeing, at least contributes to it. The first time I used it, I didn't rinse it good and had similar PH spikes for a few weeks. I always rinse it now, and yes, it's a pain in the ass if you haven't done it. They say it's PH neutral, but I'm not so sure unrinsed.
At lower PPM's (yours now are still what I'd call lower) you'll see much greater PH fluctuation, but that does seem extreme. You'll want to keep a close eye on it if it's doing that.
Lets see - is there light getting into the reservoir or shining on the nutrient solution? That could cause it I think, make sure your water/nutes aren't exposed to light somehow - no open buckets without plants/rocks in them that are being flooded that are exposed to light? These could cause that PH spike, or contribute to it.
On a semi related note - here's some good advice with PH adjusting:
Set it to 5.5 - 5.6, let it drift up to about 6.1-6.2, then reset it to 5.5-5.6. Some people try to manage PH to a specific number, but the plants will love you if you let them have that range. Some nutrients are more available at the lower end, some at the upper end. 5.8 is the sweet spot, and by doing as I'm recommending you'll spend a fair amount of time around that number. But, I guess what I'm saying, is you still want to let it drift through the range, don't peg it there. (Don't try to keep it at 5.8 for example).
