I know these things are pretty voltage sensitive to begin with and low voltage creates all kinds of problems with either chattering or not pulling in at all, but we have a controller that has good voltage (129-130 vdc) and we get intermittent relays that don't want to pull in, usually after the car has sat for a bit. This new job to us just had a pump and pump motor failure (pump broke and locked up) so we installed a new power unit. When we fired it up on automatic after setting the valve up the car would intermittently sit there and go down then up, down then up. Found the DLV relay not pulling in, but if you helped it with your finger it would run fine for 20 trips and then do it again. Changed out relay and it seemed to fix problem, but now the S relay is doing it too. Trust me, all contacts have been cleaned, voltage is good, and it is very intermittent. I believe the DLV relay that wasn't pulling in is what caused the pump failure due to the car sitting in one spot and going up and down for probably hours. Time for a controller mod if you ask me but you already know what the customer is going to say after we just installed a new power unit.