Thanks for the help guys. Unfortunately it is a P0300 - multiple misfire code, and no other (e.g cylinder number). I think Ill pop the hood open tomorrow and check for vacuum leaks.
How many miles are on this 5.7?
Have you ever replaced the spark plugs?
Have there been any major mechanical repairs since you became the owner?
Like the earlier poster said, I would get the spark plugs out and record their condition and plug gap.
With a P0300 Multiple Cylinder Misfire DTC and no other single cylinder misfire DTC's set, the best way to determine if you actually have a misfire is with a good scan tool. With a scan tool we could look at the misfire monitor and see which cylinders were actually misfiring and record the RPM at which the misfire occurs.
Some other things that you can do to determine if you have good cylinder integrity is to perform a compression test and a cylinder leakdown test.
A compression test can only tell us so much. A cylinder leakdown test can tell us if we have a mechanical problem with the cylinder itself i.e. piston/rings/cylinder wall or if we have leakage at the intake or exhaust valves on a particular cylinder.
You can rent a leakdown tester from your local auto parts store. You will need an air supply like from an air compressor or an air tank to perform a leakdown test.
