Ph Problems in ebb and flow

Hello all i have a 40 gallon botanicare res with a 3x3 table my problem is when i ph my res i ph it to 5.8 but whenever the table floods and i take a ph reading from the table the ph has shot down to 3.1 i was wondering what was going on? i am using GH 3 part with Florablend and Floralicious plus