Ok, that is very limited info, but crashing/restarting during high load could point toward either overheating issue either bad PSU.
Use AIDA64 or other monitoring program to check your idle/load temps and voltage.
if its the PSU which is a cross-hair 750W would it still boot and run propery at normal load, and how can i be sure its the PSU which is at error. and afraid of buying things that are not the problem
Like i said... check your idle and load temps and voltages. 12V, 5V & 3.3V are the voltages to keep an eye on for the PSU. CPU core temps also. All this can be done using AIDA64.