I have a friend who has a MK6 tdi. After sitting over night it'll crank but not start. If you spray just a little starting fluid in it, it will fire right up and idle as long as you let it. It revs fine in neutral. When you drive it it won't rev past 2,000-3,000 RPM. Up until that RPM range it doesn't feel like it's lost any power. Feels like limp mode to be, but the glow plug lights don't flash. Any ideas? Any and all help is appreciated.