I just went through this problem a month ago, p0300 was the code that I was constantly getting and every time I cleared the codes I would get different cylinder misfires, after about 2 months of reading on the Internet I found out the early n62 engines have an intermediate lever issue (part of the valvetronic system) I literally did everything you wrote (plugs, coils and even changed the ecu) but my indy replaced my eccentric shaft and intermediate levers and it fixed my problem.