Check for codes.
Get paper and pencil to mark down flashes and pauses of Check Engine Light.

Cycle ignition key On OFF On Off then leave in On

Watch and count groups of light flashes and pauses
or see if digital odometer readout changes from miles to a CEL code readout.

1999 46RE auto trans has Variable Fluid feature controlled by PCM computer that should be a suspect.

Check fuses and resets