Pump Continuously surges

I work for a leading chemical company and this problem has haunted 2 Territory Managers trying to fix this issue. In the account we service the motor 2 to 3 times a month sounds like it is surging and that there might be air in the pump. Once we remove the pump and reinstall it all is well surging stops.

Here is what we have already done.

1. Checked the incoming building water pressure which is static and comes in at 20 PSI.
2. Augered the wash arm manifolds to check if there's anything causing the air gap inside the pump housing.
3. Made sure all wash arms are free of debris.
4. Nothing is ever caught in the pump or manifolds.

This customer is getting extremely angry with this machine he's purchased. The only thing we can think of doing is drilling a 1/8 size hole in the impeller just in case air gets caught so that it may travel in back of the pump seal. Since Jackson Tempstar motors have the motor impeller static you are unable to move the impeller forward of backwards to help with pressure.

Can someone offer a solution? We are getting tired of constantly removing the motor check and re-installing.

Thanks