I have heard of this problem many times. Vaporlock could be the problem. As the heat radiates away from the engine during normal operation, it can turn the petrol to vapor. Are there any heat shields in place. Are there exposed fuel lines? If so, puchase some asbestos sleeving and cover these exposed lines.
Another problem could be a non-vented petrol cap on the tank. Try running the mower with the cap loose. See if this makes a difference. Good Luck!