stalling after 5 min.
 

1994 Mercedes sl500 150k. 5.0 eng. Car was running hot Never over heating.. I replaced the water pump. Started the car and let it idle and it shut off in 5 min. 80 deg. temp. Let it cool and the same thing happened. Is there a temp. sensor that may have gone bad. Am I looking in the wrong area. The only code that comes up is and old code. Mass air flow sensor.. Need some help ///

loose belt? clear the code?
 

Your belt might be a little loose. I've been told that the engine controllers monitor belt slippage. After a certain point it shuts the engine down to prevent damage. Re-check to make sure that you have the correct tension, but don't over-tighten.

Otherwise, it might be just a coincidence. Even after you've cleared the code and cleared the memory (disconnect the battery for 30 seconds, turn the key to on power on position ,but do not engage the starter, for 10 seconds, turn it off for 10 seconds) only the same code comes up?

Not true.