ph low

Just to add a word to Watermom's advice, I would also bring the alk up to the 120 range. When using trichlor, the higher alk will help keep the pH from swinging so far and so fast. The fact that you had high alk before is probably why you didn't have as much problem controlling your pH as you do now, with lower alk.

Watermom thanks for the suggestions, we use a pH meter to check the pool, it was a cheap investment to get a much more accurate reading.

Aylad with that in mind maybe we will give the pool a little dose of soda. I have forgotten when you want to up the pH but not the Alk do you broadcast or put it in the deep end?

Sal soda will raise both pH and TA. It does not matter if you broadast it or put it in one place. You are thinking about the 'slug method; of lowering TA, which has been proven NOT to work. All slugging acid does is ruin the pool because of the pockets of very low pH created.
Baking soda will raise TA and cause a slight rise in pH compared to sal soda
Borax will raise ph with minimal impact on TA.

As far as your pH meter, are you calibrating it on a regular basis and storing it with the electodes wet? pH meters are only accurate if they are regularly calibrated against standard solutions and if the electrode is properly cared for. The ONLY kind of pH meter I would trust is one that uses a 3 point calibration (calibrated against 3 different pH standard solutions.)