I have a 2000 320 4matic wagon with 110k miles. At highway speed 75 mph, check engine light comes on and car runs rough. When I stop car, and restart, engine runs fine again. Transmission shifts abruptly when starting up again. This may happen just once on a long highway trip. Otherwise, engine and trans perform well. I have replaced plugs and coil. Suggestions?

What you're describing is a condition known as a serious misfire. The ECU is actually disabling certain cylinders to prevent damage. In other words, it's running rough because you no longer have a six-cylinder engine. When you shut off the engine and restart, it re-enables those cylinders.

The first thing to do is to check codes. Once you have codes, you can start a methodical approach to diagnosing and repairing the issue. Until then you are just guessing. Some auto parts stores will run codes for you, but be sure you get the actual codes, not just their "diagnosis." Or you can buy a basic little code reader for $16 on Amazon, the MS300, it's a good little unit.

Now, if you want to swap some more parts without actually diagnosing it, a place to check/start would be the air filter (good quality unit like Mann) and be sure the airbox and tube are clear, you may have a nest in there, it happens. Front O2 sensors are also a possibility, as well as the MAF and the intake seal at the TB (from the intake tube).

Hope that helps. Post back with codes after you get them.

Good luck.

Did you replace plug wires as well as coils and plugs? The wires are known to degrade on these cars starting at 7-8 years of age. The problem first manifests as an occasional less smooth idle, but will progress to an outright misfire. It typically manifests more on rainy/damp/humid days.

My car did that a lot, I replaced all 12 spark plugs because they were worn down really bad.

New NGK platinum plugs and never saw that prob again. Taking off the wires from the plugs will require a special tool like a 17mm wrench or something similar and only use a slim spark plug socket.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)