It is partly true that the Borax will buffer the pH so that when you do add acid you will be adding more than if the Borax were not there, but as waterbear points out there are reasons why the total amount of acid needed will be lower. Borax has a double-effect -- it's both a pH buffer AND an algaecide and the latter allows for less chlorine and less chlorine means less SWG on-time and less pressure on making pH rise.

Your TA will eventually drift downward and the pH will slowly drift upward. Anyway, if you CAN test your TA (and CYA level, if you can, though TA is more important), that would be helpful -- not to solve any problem, since there isn't any, but to help confirm or refute what I believe is going on. I always want to compare the "theory" against real-world behavior.

Thanks,
Richard