What is wrong with 8.4+? I've always read that the recommended high limit is 8.5 Is your alkalinity in meq/lt or dkh? If its in meq/lt its fine. In dkh that is really low and doesn't make much sense.
Are you using a test kit or a probe to test your pH? If its a probe, how old is it and when was the last time it was calibrated? Have you double checked your pH with multiple kits/probes? Why do you even monitor your pH?... seriously, the only reason I do is because I run a calcium reactor and kalkwasser reactor which can both have potentially negative effects on my pH if something goes wrong. For the rest of the population its unnecessary in my opinion.
This might help too
High pH: Causes and Cures
http://reefkeeping.com/issues/2005-03/rhf/index.htm