I have a 99 Dakota with the 5.2L magnum (250k miles)that I just picked up a couple months ago. First thing I did was install new plugs(Autolite 5224) serpentine belt and air filter. Ran great with no codes. Recently it threw a P0303 so I installed new plug wires, then new cap and rotor. I swapped #1 and #3 injectors to see if it was the injector but I still get a P0303 code. Doing a compression check #3 cylinder has 120 psi and I have no vacuum in the crankcase that I can tell so I wouldn't think it would be the valley pan gasket. There is a noticeable mis coming from the exhaust pipe and at idle and the spark plug is grey ashen which tells me it's running Lean.

Any suggestions where to go next?

 

You have a thread in the Dakota section, which is what you are driving. This is the 2nd gen RAM section. Not Dakota.