Sounds as though it could be vapour lock or possibley a fault return valve in the pump. One easy way to check if it is fuel is to pulse the pump when you have the problem. Ie if she wont start, don't crank the engine, just switch on the ignition, wait 5 seconds then switch off, repeat 4 or 5 times then crank the engine. Each time you switch on the ignition the fuel pump is pulsed for a few seconds and so if done in sequence it will prime the fuel system and build the pressure up. If it works, next time reduce the times you pulse the pump, until you find the minimum number of times required. Dont bother cranking as all you do is waste the battery. To check for vapour lock/valve fault you need to have a fuel pressure guage fitted and recreate the conditions which cause the problem.