soudns like a carb problem, check the anti-dieselling solenoid on the carb (the bit with a wire going to it)
they tend to bung up or just fail and cause the mix to lean way too much, if you hold about 2krpm+ it doesn't cut out though, if this is the case, then check that valve out.
you can , to test, remove it and then pull the end bit and spring off the valve and replace without the parts, race cars did this due to the potential failure.