I am not 100% sure, but does the 3.5l engine have the same water pump as the m50 2.5l ?
I had a similar experience and it was due to the water pump. The impellor is plastic and is a push fit onto the drive spigot. I found that when the engine got hot, the joint expanded slightly and started to slip, leading to the engine hitting the red zone. On stopping and cooling down, the plastic contracts back onto the spigot and all is well, until the next time.
Not sure if this is the case with the 3.5 engine but it would give the symptoms you are having.
HTH.