Yep, it has only done it so far (sometimes) when I go over 3500rpm (once the car is warm).
I think it has only been happening when I cruise at that speed, if I am accellerating I can go past there without a problem.
Yep, is has the standard computer (standard for the motor, not the car = JZZ30 1JZGTE Soarer).
I don't think that testing the spark plugs will do anything as it is an intermittant fault and only does it at rpm & speed.