I've been getting misfires on my 1997 E320 (80k miles). The engine seems to run fine on the freeway, but runs rough

waiting at lights. The RPMs also surge, especially if it's in park.

My AutoEnginuity tester shows a variety of codes. Usually P0300 (random misfire), plus P0301-4 (misfire cylinder

1-4). Here's what I've done, in chronological order:

- Action - DTC
Initial problem - - P0300, P0303
reset DTC - Replaced all plugs with Bosch F8DC4 - P0300, P0303, P0304
reset DTC - Swapped coils on cylinder 2 and 4 - P0300, P0303, P0304
reset DTC - Swapped plugs on cylinder 1 and 3 - P0300, P0303, P0304
reset DTC - Swapped plug boots on cylinder 1 and 3 - P0300, P0301, P0304
reset DTC - Replaced 3 plug wires & 6 boots Bosch 09386 - P0300, P0301, P0303, P0304

This is very frustrating - the problem seems to be getting worse with each part replacement, and the parts left to

replace are getting expensive!

Any suggestions?

Some possibly related information:

1) I replaced the head gasket (and all the vacuum lines) about three months ago. This misfire problem started in

the last 3 weeks, so I don't think they're related. The wiring harness looks OK, although I have not unwrapped the

harness to check the wires in detail.

2) I've been having a P0170 - Fuel Trim DTC off and on for over a year. It takes about 1 month to pop up after a

reset, so diagnosis is slow. I've replaced the Mass Air Flowmeter, the fuel header pressure regulator, and the

injector o-rings. Fuel header holds pressure for > 30 minutes after motor turned off. Injectors don't drip.

Thanks.