a scanner to check the fuel trims while acting up would be best! Vacumn leak would probably show up at idle also though. Surge in idle or fords like to shutter almost take off at idle while your on the brake with a vacumn leak. I bet its plugs though. Is there a slight delay while cruising and you load it up with heavy throttle? I would just pull the plugs and check them first, bet thats where you will find your issue
