engine misfire on acceleration
We have owned an 09 Enclave CXL for almost four years. The car had 69,000 miles when we bought it, it now shows 135,000. The problem we have now, that did not change after the throttle body and throttle position sensor were changed on recall, is the engine picks up a misfire just prior to a downshift at highway speeds, in cruise control or not. Getting ready to replace all spark plugs and coils but problem existed at 75,000 so don't expect any change. Buick dealer did a check once but found nothing. Any possible known causes?