oil pressure/1990 MERCEDE300 SEL

I HAVE A BIG PROBLEM WITH MY BENZ 1990 300SEL IF I STARTED THE ENGINE IN THE MORNING THE OIL PRESSURE WAS GOOD THE READING AND IT GOES TO 30 PSI BUT IF IT IS HOT THAT THE WATER TEMP. GOES TO 80 f. THE OIL PRESSURE GOES TO 0 PSI AND NO READING AT ALL AND NOW THAT IS THE TIME TO KNOCK THE VALVE ON THE TOP OF THE ENGINE AND IF I TRIED TO MAKE IT COOL FOR 30 MINUTES THE ENGINE AND THEN I WILL START THE ENGINE AGAIN THE OIL PRESSURE READING WAS GOOD AND YOU CAN HEAR A LITTLE KNOCK SOMETIMES ON THE TOP THAT MAYBE THE VALVE.WHAT WILL I DO FOR THIS? I TRIED TO CHANGED MY OIL BUT ITS THE SAME THING.ANY SUGGESTION WHAT WILL I REPAIR OR ANY HELP THAT YOU CAN GIVE ME. I WANT BY MYSELF TO REPAIR MY CAR.THANK YOU SIR