Hi
You may aleady have solved it by now but I just found the site . If it helps any I had a similar problem. It would always start and go but a few miles into it the engine would start to misfire and gradually get worse to the point of being unreliable .I never loaded it up to where it would stall. After several Dealers, one of whom told me the engine was shot and there was hydrocarbons in the coolant I found a local shop who pinpointed it the PLATINUM plugs had reached their usable life. Apparently platinum plugs don't show the waer on the tips ( I looked) they break down internally from heat and load and start to misfire. I put a change of plugs in and haven't had a problem since.
Phil