2005 chevy classic cranks but wont start already changed ignition modual what next has good fuel pressure please help

If it cranks good but won't start, have a helper crank it while you visually check for spark. If spark everywhere, use a gage and check proper fuel pressure and fuel injector pulse. Any applicable trouble codes?

Lets start at the beginning.
Do you have spark to the plugs and normal fuel pressure ?

Check your fuel pressure regulator,sometimes it does not hold the pressure, also check the fuel pump for proper operation. Try to change fuel filter it could be clogged...

I just have one suggestion, after what all your mechanic already did. I would want to know the fuel pressure reading WHEN IT WON'T START. So you would need a fuel pressure gauge handy, for that time it won't start. Hook up the gauge and turn the key on- if at 59 psi, when the engine won't start, then you know the fuel pump and the relay sensor is not the problem. And so probably not a fuel problem, although you haven't checked the injector circuit, but likely that wouldn't exhibit a problem as you have.
So if not a fuel problem, check for spark when it won't start. Maybe the problem is in the ignition system. Buy a spark tester from a parts store, use it when the **** engine won't start- simple and easy check of your ignition system.

you should check the battery, if it is all right, you should try to check the starter and ignition.
_____________
Launch X431

The next time it does't start check for spark at any of the sparkplugs if you have no spark then,check the crankshaft sensor for a signal if its good then its either the igniter or the ignition control module.

I have a 90 chevy k1500 350 tbi with same problem! I have traced problem to... after turning on key fuel pump kicks on when u crank it over injectors shoot gas and truck starts! After few seconds injectors stop spraying and truck dies! This can happen many times over and over! Eventually sometimes truck stays running and all is great! I changed ignition module, oil pressure sending unit (which i was told controls gas after vehicle is running) fuel filter, computer, fuel pump relay, I am at a loss now! Hope this helps and hope someone can help me!

clogged air filter can make a car hard to start also.leaking fuel injectors flooding out engine.after turning off.take a fuel pressure gauge hook it up.turn ignition on .dont start engine.let your fuel pressure build up.55-62 psi.then turn off ignition.if fuel pressure start leaking down.your fuel injectors are leaking. if they dont leak. you have problems in the ignition system.

Spark when cranking? Possible bad sensor

Most likely it is the ignition module or the crank pickup.