Do you bubble your tap water overnight before mixing/adding to your res? I have found that by doing that, my pH stabilizes after about a day at the most....usually, if I let it go the full 24 hours, it will hold its pH in range for days after the initial adjustment. I just keep a bucket of water with an air stone or two in it all the time. I don't know the science behind it completely, but I have read that by letting the chlorine escape the water, the pH won't have such swings. The airstones just help it along, it will even happen by just sitting the bucket in the corner overnight.