Could it be a ecm problem

Without a lot more information, observations, or test results to analyze, your best bet is to have the charging system tested. If the problem is intermittent, the testing has to be done while it is occurring. Computers are sensitive to low system voltage and will do weird things. If they shut down, they will turn on their warning lights to tell you.

Just today driving my saab 900 s the gearbox light lid up and the car lost speed and power

Did you have the charging system tested?

Battery and alternator check fine

Was the problem occurring while the tests were being performed? If not, the tests have no value. If it was occurring, what did the mechanic have to suggest?

The battery and altenator check out fine computer codes showing 2 o2 sensor codes and a system voltage malfuction code taking when my car stalled with a check gear box light on.

You aren't telling me anything I can use. I need specific charging system test results to analyze. That means specific numbers while the problem is occurring. "Tests okay" is not something I can analyze. Those numbers are especially important in light of any fault code related to low system voltage.