Are you now getting a PO301 code rather than PO305 as before? There is also a TSB on this vehicle related to misfires typically on cylinders 3-4-5 or 6 because of a valve stem binding.
I had the misfire on cylinder 5 (PO305), I replaced the SCPI with MFI and it didn't help.
Here is the TSB:
Subject: SES Light and P0300 When Towing, Cruising Uphill or on hard Acceleration - kw 4.3, 5.0, 5.7 accelerate cruise cylinderhead DTC L30, L31, L35, LF6, LU3, P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307 P0308 # PIP3081 - (07/23/2004)
The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.
The vehicle may exhibit a SES Light due to a P0300 and misfire. If the misfire is related to the information below, it will typically happen while cruising uphill, pulling a trailer or on hard acceleration and then stop misfiring shortly after returning to an idle. Typically, cylinders 3, 4, 5 or 6 will be the ones to experience this.
If the P0300 SI diagnostics did not isolate a concern, the following may help:
Remove the valve cover, valve springs and valve seals on the effected cylinder or cylinders. A small wire tie or rubber bands can be placed in the valve stem keeper groove to prevent the valve from falling into the cylinder. Rotate the valve while moving it up and down in the guide to see if it binds. If a binding valve is found, remove both cylinder heads and use either of the following methods to increase the stem to guide clearance:
^ Send the head out to a machine shop and advise them to increase the valve stem to guide clearance to 0.002 inch per guide.
^ Hone the guide with a 9 mm hone, such as Snap On BCG249, until a clearance of 0.002 inch is obtained. This will usually take about 4 strokes of the hone.
Reassemble the cylinder heads using new valve seals.
The easiest way to possibly correct this condition, if indeed this is your problem, is to change to a light synthetic oil (5w 20) and add Z-Max micro lubricant with a premium (K&N, Bosch, etc.) filter and drive. This will lubricate the valve guide much better. I did and the problem with the PO305 misfire was gone.
Another common problem is the lower intake manifold gasket leaking, either/or internally or externally. If you have ever needed to add coolant, this could be a problem as well.