Any one of those things could cause it. However, the hard start makes me think fuel. When this happens, have you ever tried to check fuel pump pressure? Also, when it becomes hard to start, have you ever tried to give it a shot of starting fluid to see if it starts right back up?
Again, based on your description, it sounds fuel related. I realize you replaced the fuel filter, but the pump may not be able to keep up at that RPM. As a result, the system pressure drops and it makes it hard to start.
I would recommend starting with that. Let me know what you find. Here are the instructions for checking fuel pressure. Let me know what you find.
I am assuming that the check engine light hasn't come on since you didn't mention it. Usually an electrical / sensor problem will cause it to come on.
Monday, March 8th, 2010 AT 3:14 PM