2000 E320 Misfire codes

[PatOpsahl]

2000 E320 Wagon 4Matic, 142k miles.
Took a trip this weekend and the check engine light came on while going up a hill and transmission was downshifting. The engine power went way down and we stopped at the next exit. I have scanner and read codes: PO 305, P0306, PO304, PO300 - All misfire codes. Errased the codes. Drove the next 100 miles and it happened again, going up a hill. Same codes.
The next several hundred miles was on flat ground and it ran OK. Then it happened going down a hill, codes PO302, PO301, PO300. Erased the codes. Finnally got home after having to erase the codes going up 3 hills and down 2, 5 times in all.

Going up the hills heard the motor pinging before getting Check Engine Light, like it was lacking higher octain fuel - we always put in Supreme Unleaded.

Before the trip I replaced all 12 spark plugs. The plug wires looked good.
The missfire codes started happening after driving 1500 miles trouble free.
The misfire codes happened an all cylinders one time or another on the trip.

Could all my coils be going at the same time?

I was thinking fuel injectors need cleaning?

If my Cats were plugged, could it cause back pressure triggering the codes? I have always had an annoying rattle that I think comes from the Cats or muffler.

Please help, Patrick