P0301 - can't get rid of it
 

If you've got P0301 (front rotor misfire) on a 2007 231 (20,000 miles) and changing coils, leads and plugs doesn't get rid of it, what would you advise to do next (if you don't want to throw money at a dealer)?

In more detail:

Throughout the engine has seemed perfectly normal all the way up to the red line (this is my 3rd RX so I know what 'normal' is, even if my ex-wife tells me I'm not).

When first the CEL came on I had code P0301 (1 full, 1 pending) and on investigating I found that one of the coils had an open circuit. I changed the coil and all was well for 100 miles then when I used high revs the CEL came back on......

This time I changed the other 3 coils, the leads and the plugs. Again, all was well for a few miles then when I revved to 8,000 rpm the CEL came back on. I then found that one of the plugs was loose, tightened it up and all was well for a mile or 2. Then I revved high and the CEL flashed - yep, code P0301 again.

This time I checked all the work done and swapped over the coils between rotor 1 and rotor 2. Revving high led to the P0301 coming back.

The manual lists causes other than coils, leads and plugs but befoire I start making work for myself I thought I'd see if anyone here can help.