blackwolf wrote:

The camshaft position sensor is also critical when starting (since without a reading fro it the ECM cannot determine which stroke the engine is on), once the engine is running it is not used. I don't think you have a camshaft sensor fault though because I don't think it would fire at all if this one wasn't working.

It could be a weak signal from the crank position sensor such that the ECM is only picking up something recognisable above a certain engine speed, or it could be a fuel pump problem whereby there is insufficient pressure below a certain speed. It could be a VCV problem although I think this is actually less likely in this case. It could be that the PRV on the rail is leaking so that fuel rail pressure can only build up at higher engine speeds.

If I was investigating, the first thing I would be checking the fuel rail pressure when cranking, since this will help narrow down the problem.

Note that if the crank sensor is the problem I would expect normal fuel rail pressure when cranking, but the ECM won't be firing the injectors. If the PRV is leaking or the pump is faulty I'd expect low rail pressures and the ECM will be trying to fire the injectors.