started when i drove on interstate yesterday.. after cruising 80-85 mph yesterday started to shift hard every upshift.. smooth on downshift.. no slippage.. turned off truck to go into store no more then 5 min.. no problems when i restarted it.. stop and go traffic in town but when i got back on interstate after 30 min or so started hard shifting again... no problems later that evening or this morning.. what could it be.. time for a service?