Saw a similar occurance last night.
Was in my P-51D at about 29K after being AFK. When I get back, I see an A6M off my 6 nearing 500 yards.
I leveled off and started to pull away from him. Once I got to about 3.5K, I figured I would turn back into him and easily outfly him.
To my suprize, the A6M was able to turn inside me twice and then still have enough energy to manuever back onto my six.
Figuring a fluke, I extened again and this time got to about 5K before going back for him. Again, the A6M out turned my P-51.
I went into a dive after a spit, and the A6M stayed right with me, never getting further back than 1.5. at 20K I pulled back up figuring to leave the A6M lower so I could go back on him from above. Again, the A6M stayed with me and we ended up at CO-E states again.
The A6M pilot (Damned Felix) and I had a friendly conversation about it, as did a few other people in the arena (Whels and lazzer)
Eventually, the A6M ran low on gas and went home. I was very frustrated that I could not get an advantage on him at 25K, where the P-51D is supposed to shine.
I found it odd that an A6M would be able to maintain enough speed and manuverability at such a high altitude as to prevent a P-51 from getting a position advantage.
All things were check on my P-51 during the flight. Flaps up, gear up, full RPMs, full MAN pressure, centered trims, no externals.
I consider myself a fairly decent P-51 pilot and know how to manage my E state pretty well. Strange is all I can say.