mercedes 1990 420 sel

[sam44]

Bought my son a 420 sel. Ran when we bought it this summer. Changed plugs, wires and 02 sensor, fuel filter. Ran great for 2 months and then would not crank. towed home. Found out it was fouling plugs. Got it started, black smoke boiled so I put another fuel distributor on it. Cranked up and ran without the black smoke. A little rough since I had not changed the last round of plugs that fouled. Ran for about 10 minutes. Turned off , would not crank. I changed plugs today putting in new bosch. Took about 30 seconds of turning over but got it cranked. Ran with a little more black exhaust. Ran 10 minutes. Would not crank. Pulled brand new plugs. fouled. Fuel is dumping. Why. to recount new filter, fuel pumps, distributor, wires, 02 sensor. Have checked pressure regulator for fuel coming out of vacumn tube on back side of regulator. non leaking. Any guidance on why plugs are fouling is appreciated. Thanks. Sam