When we originally built this circuit we were experiencing a lot of negative generated spikes coming from the power relays that we were switching inside the HVAC unit. We combated this with a 51V Bi-Directional TVS. I now starting to wonder it maybe we should be using one in the range of 12-15V instead. This might be source of or problems. Out of about 1,000 units about 20 of them experience this op-amp problem. Most of them seem to be job specific so I'm thinking it might be a combination of things inside the units that are making this happen. It's become quite frustrating. I've tried to duplicate the problem without any luck though.