If it continues to loose coolant and you don't see a leak on the bottom or ground, you may have a "plenum" rotted/burned thru at the EGR port into throttle housing. It you remove the throttle opening screen and maybe the MAF sensor also, hold/prop the throttle wide open. Get a good flashlight and look closely at this egr port where it comes thru the bottom of the intake port to throttle. This is where they usually leak. If the system is cold, warming the engine first will build pressure to help expose the leak. This type of leak won't leave spots on the ground or chassis and it definitly makes them run crappy and use coolant. I've seen a few that blew the plenum ports and got soo much coolant inside they hydro locked(filled a cylinder up).
Otherwise, do the normal checking of plugs, wires, fuel filter, MAF sensor operation (you can unplug this and run it to see if it runs better), etc).
have fun........... Jake
