Starts easily runs then when warm runs rough and stalls

Hello and thank you very much for any help you can provide. My 3000gt starts and runs, however then it begins to run rough and may stall. The car is stock except for a K&N filter. New tune up, o2 sensors and coil. The dealer checked all fuel and electronics and could not find anything wrong. Been very frustrating, truly appreciate your help.
Best regards,