Engine dies when warm

My son has a '90 Laredo that is giving him fits. It runs great early in the am when the weather is cool. By 11-noon the rpm's flutter as it struggles to stay alive. The engine dies and will not restart immediately. If he unlatches the airbox to let some exhaust gas out it will start and run for a short time, the rpm's flutter again and then dies. He has replaced all the vacuum hoses, thermostat, power brake booster and fixed an intake manifold leak. We really could use some adivse on what this might be.

My son has a '90 Laredo that is giving him fits. It runs great early in the am when the weather is cool. By 11-noon the rpm's flutter as it struggles to stay alive. The engine dies and will not restart immediately. If he unlatches the airbox to let some exhaust gas out it will start and run for a short time, the rpm's flutter again and then dies. He has replaced all the vacuum hoses, thermostat, power brake booster and fixed an intake manifold leak. We really could use some adivse on what this might be. HI, WHEN HE OPENS THE AIRBOX HE IS LETTING FRESH AIR IN-- CHANGE THE AIR FILTER , CHECK MAKE SURE THE IS NO RESTRICTIONS IN AIR BOX,CHECK CCV HOSES CLEAN CCV VALVE,TUNE UP, PLUGS ,PLUG WIRES, DISTRIBUTER CAP, & ROTOR HOPE THIS HELPS. DALE [email protected]