1991 olds 98 electrical problem

Battery warning light(battery volts high) voltage varies from 12-18 volts. checked ground and battery connectors(checked out o.k.). Continued to get additional messages on information center(coolant sender problem, battery volts high,hot...stop engine). Also, check engine light will come on then go off. At times engine will run fine and voltage will be fine. There are times when engine will shut off when at an idle and sometimes car will not have power or shift properly. I had battery and alternator tested and both tested o.k. I have since changed alternator and still have same problems. Could I have and ECM going bad?