When I crank up my 2003 Chev Suburban Z71 (or while driving) the “BATTERY NOT CHARGING” message shows up on the DIC, and once in a while the “SERVICE BRAKE BOOSTER" message will follow. This is random, and usually starts cycling off and on for 20 or 30 minutes, then it might quit for the rest of the drive. After stopping at a destination, it will then restart after I crank up again, and the same thing happens all over again. Odd that it is intermittent if the alternator is bad. Could it possibly be Powertrain Control Module going bad? Any ideas? Would the alternator show "bad" if the Powertrain Control Module was failing? And if it is the Powertrain Control Module, is it a difficult task to replace it? Sure do appreciate an opinion on this...I'm just not able to afford to start randomly replacing parts that don't need replacing, and would really like to know how to get a handle on the problem. Thanks all.