Rising pH??

My pool has been crystal clear this year after following the BBB method. I have been extremely diligent in my testing with my Taylor 2006c kit. One problem I have noticed, today as well as about three weeks ago, was an unexplained spike in my pH. The ONLY thing I have added to my pool for at least 6 weeks is liquid chlorine and that rise in pH went unexplained. I added acid and all was fine. Then yesterday, the same thing happend. My pH was well over 8 and this morning I added 1.5 qts of acid to bring it back down.

The other thing that accompanied those two rises in pH was that my cc was at .5 both times. The rest of the summer my cc was perfect, always getting a 0.0 reading. Is there something going on here? What woud cause my pH to rise like that? We have had no rain at all, either. Absolutely no other chemicals added, and very LOW bather use during this time. Water crystal clear.

My other numbers are as follows:

chorine - never below 9 ppm
pH - stable at 7.4 (except the 2 times it spiked on me)
TA - 110
CYA - 70
Hardness - 280

What is your chlorine level when you are testing pH. High chlorine levels can give falsely high pH readings.

My chlorine was 9.5 ppm this morning when the pH was well over 8. Yesterday morning my chlorine was 9.5 ppm and my pH was 7.6. I added nothing but maybe 1/4 gallon of chlorine last night. The same thing a few weeks ago....just a big spike in pH. It wasn't even a slow rise, just a big spike from one day to the next. I also tested a few times and from different areas of the pool just to make sure it wasn't a testing error and reading artificially high. Also there are no water features that would aerate my water, other than just the filter running which causes slight rippling in the surface of the water.

At some point this summer (there have been so many posts this season I can't begin to remember who the poster was) there was another post similar to this one, and the issue turned out to be residual indicator left in the test block. Are you careful to thoroughly rinse and dry your test block between tests?

May or may not have anything to do with it, but it's the only suggestion I can think of at the moment....