2007 Hyundai SONATA electrical

When taking off from a stop sometimes will lose all power(voltage) to everything(ECM,lights, ect)for about 2 seconds then it all comes back on and off I go.Have triple checked all battery connections and grounds. It keeps resetting the ECM readiness monitors so makes it hard to get a smog test done. Has anyone come across this issue before? Doesn’t do it sitting in the drive rev’ing motor only when beginning motion and last only a couple seconds then goes away and everything is normal so makes it really hard to troubleshoot.