This uses a " waste spark " system where each end of a specific coil has a spark plug. When one cylinder has an a/f charge on the compression stroke, the other is on the exhaust stroke both plugs fire.

Going from memory, the pairing should be 1 / 6 , 2 / 5 . 3 / 4. Is your 1 / 5 pairing a typo?

Be aware that just changing spark plugs can temporarily mask a failing coil so first swap coils and see if the problem follows. You don't have much choice to move plug wires around so use an ohm meter to test.

Another oddity I haven't figured out. At least in the aftermarket, the 97 C280 takes a different coil part number than the 97 SL320 / 97 E320.( or some similar odd split ) Both use the same basic M104 engine and both use the Bosch ME 2.0 / ME 2.1 computer.