New water pump gasket leak

Water pump gasket failed at 104,000 miles. New water pump was supplied with urethane o-ring gasket. Applied small amount of silicone o-ring lubricant. The unit fit snugly but not too snugly in the block cavity. Filled with water to check for leaks. Overnight, significant leaks observed at 9 to 12 o'clock position. Removed pump and closely examined mounting surfaces. Some residue remained in o-ring groove, but also, the groove disappeared at the 9 o'clock to 1 o'clock position. Only flat, smooth, clean iron at this position. Thoroughly sanded and cleaned groove and flat area. Reinstalled using thick, heavily applied silicone o-ring grease. No leaks overnight. Started motor and observed leak at 9 - 12 o'clock position. Leaks stopped after four or five minutes running motor. Idled motor for five hours with chemicals to flush rust. No leak. Drained and refilled with water to flush out chemicals. Leak observed again for four minutes, and then, it stopped. Close inspection now shows periodic leak of several drops appearing at 9 o'clock position after motor is warm - not enough to accumulate and flow to the ground. Cold starts always create a five inch diameter wet spot on the ground. My tentative conclusion is that the o-ring groove gap in the block is a manufacturing defect. Any ideas for a solution. I am taught and believe gasket compound must never, never, never be used on an o-ring seal because (1) it interferes with the proper operation of o-ring sealing dynamics, and (2) it could make future removal of the water pump impossible.