I HAVE BEEN HAVING MY 96 SLS STALLING ON ME, AND THEN HARD TO RESTART. WHEN IT DOES START WHEN I PUT IT DRIVE AND TRY TO EXCEL IT DIES OUT.RECENTLY I PUT SCANNER ON IT AND THE CODE IS P1406

 

if you have put the OBD2 scanner the this is what that code means:

P1406 Heated Catalyst Internal Control Module Checksum/ROM Error
So what you do from here I am not sure but now you know what that code meant.

Good luck.