Plaster chipping off in spa due to spa draining when pump not running. HELP

So over the past 3 months my spa has been draining back to the pool maybe 4 times a month it would do this, now over the past 2 weeks it does it 99% of the time. I have rebuilt or replaced all my valves, then I remembered the inground cleaning system and i think that is the issue, but the real problem now is the plaster in the spa is now chipping off in 2 - 3 inch spots. Does anyone have any ideas to help with the draining and or plaster. HELP