Pull the valve cover and closely examine the springs. Sometimes they break toward one end and still have enough pressure to close during cranking, thus will not show up on a compression test, but cannot snap shut fast enough when the engine is actually running.

 

You will need access to a compressor and also an adapter to get the air hose connected to the spark plug hole. The adapter can be purchased at Napa stores. Before you perform the test, you need to make sure your #6 is at TDC. NOTE: Stay away from the fan or belt when applying pressure. If the piston is not at TDC, it will crank the engine suddenly and you don't want to be caught up in it.

Make sure your valve springs are good first, as recommended above.