I add my nutrients, then allow it to mix and aerate thoroughly with a bubbler. The first pH reading is NOT indicative of the final value. There is usually some bicarbonates in the water releasing free CO2. That has to come out of the water first, and so the pH rises. I check again after ± 24 hrs and then correct if necessary. Only then does the water go into the main tank.

Your reservoir size could be too small if you see big spikes.

I'd agree with Willard3, sounds like your reservoir may be too small? What size and what exactly are you using and growing? Build your own?

My well water is 6.2 out of the tap, so its somewhat acidic to begin with. Once I slowly add my nutrients I wait for a while until my sprayers and aerator mixes it up nice and then test 1 hour later, 12 hours later and then adjust typically with PH Up since my PH tends to fall over time

I drain and refill my nutrients every 2-3 weeks, sometimes topping off with fresh water at or near the temp that's in the reservoir. I don't have big fluctuations, my reservoir is near 40 gallons (2 aerosprings joined together at max capacity)

You may also want to calibrate your PH meter, I frequently calibrate so I can get the most accurate readings...

More info on my systems along with many pictures in my gallery at the link below.

Here is a link that might be useful: http://www.TechnologyGarden.net

Thanks for the comments. My reservoir is 12 gallons of liquid, and big enough to hold about 20-30 gallons total.

I'm not sure I'm seeing big spikes. It just seems as though the water likes to return to its original pH before nutrients were added. If I put Mad Farmer Get Down in, the pH will lower, but eventually creep back up, and I have to continually add the Get Down.

6/29 @ 10pm = pH 5.38
6/30 @ 2:15pm = pH 6.10
6/30 @ 8:15pm = pH 6.09
7/1 @ 9:50am = pH 6.29

I'm wondering if this is the reason to pH balance the water to 6.5 before adding the nutrients?

I'm not familiar with the Mad Farmer Get Down. Is that designed for use in hydroponics or soil?

First thing I'd do would be to observe the pH of a set of solution that isn't in the system. Mix up a gallon or two, put an airstone in it for aeration, and then just let it run like that for 12-24 hours or so. Check it frequently to see what the pH does. That way you can determine whether the changes in your system are part of a chemical reaction within the solution itself or something your plants are doing.