Another imposrtant thing that comes to mind is that may be there is an air-leak some where and the engine is sucking in air , hence above a certain RPM, in your case , 5000, it leans and shuts down ?
The only way to check that is to close the inlet after removing carb with a piece of old cycle / car tube and metal plate and two appropriately sized bolds ( may be 5mm size) and doing the same with the silencer port / hole in the engine head and then make sure your piston is at the Bottom of stroke cycle and pumping air through the nipple in crankcase or some other way and spraying some Fairy / dishwashing liquid mixed water to see BUBBLE coming out from seams / gaskits or anyother places on your engine.
2 stroke engines during their service life can seize on you if they have a leak from where they are sucking air and going borgerline lean on you .....sometimes you would not even notice that as you think your engine is cracking good and well tunned...
If the leak is significant, it would die on you after a certain RPM is reached, beyond which it might not have fuel to keep on going...
This is my thought on it.
I would definately check the timing on this engine with any timing kit , viz. Hobbyking does one. It should be 28 Degree advance.