97 Astro every 2 months wont start

97 Astro 320,000 kms. every 2 months aprox. the starter cranks perfectly but the engine wont start, its hapening for the last 6 months, mechanic doestn't know whats wrong, no engine light on and suddenly starts and works perfect.
Any idea what can be?

Anything that intermittent is very difficult to diagnose. Unless you are lucky (unlucky?) enough to duplicate the no-start condition while diagnosing, it could be a fruitless exercise.

The fact that it does crank rules out the start relay, neutral safety switch, and battery power and distribution. However, there are so many other possibilities it could be nearly impossible to pinpoint without more input.

One thing you CAN do to help diagnose is to change your routine. When you get in the vehicle, instead of simply hitting the key and starting it immediately as most people do, pay attention to what's happening. Slow down the process and use the tools provided to diagnose the problem.

When you first turn the ignition key, turn it only to RUN (do not crank the starter immediately). As soon as you turn the ignition key on, two things should happen: The Malfunction Indicator Lamp (MIL, or "Service Engine" lamp) should illuminate for about 1/4 second, turn off for about 1/4 second, then turn back on and remain on until the engine is running. If the light does not turn ON-OFF-ON when the ignition is first powered up, the PCM may not have passed its internal POST (Power On Self-Test) routine, and will not run the engine. If you get in the habit of watching the MIL every time you start the engine, you will know whether the PCM is "ready" or not. It may make diagnosing the no-start much easier. The fuel pump should turn on for two seconds to build pressure in the fuel rails and injector pod. If the fuel pump does not run this prime cycle, that may indicate a problem with the PCM, fuel pump relay, pump wiring, or pump itself. If the engine is cranked in this mode, the pump may not run until oil pressure from cranking reaches 7 PSIG, closes the auxiliary oil pressure swithc, and powers the pump. The problem with this is that during cranking, battery voltager isx usually below 10.5V, which may be inadequate for the pump to produce enough pressure to operate teh poppet injectors in your CPFI system, You could crank "forever" and never get fuel, even though the pump is actually running. Once the MIL has performed its "bounce" and the fuel pump has run its prime cycle, turn the key switch to crank/start the engine. While the engine is cranking, all thewarning lamps on the instrument panel should illuminate, and the coolant temperature gauge should sweep toward afull scale (hot) reading. If the lamps do not illuminate in this test mode and/or the temperature gauge does not indicate full, there may be a problem with the ignition switch or power distribution. Such a problem could also prevent the injectors and/or ignition system for getting power and running the engine. Once the ignition key is released to allow the engine to run, the dash warning lamps should go off, the MIL should turn off, and the temperature gauge should indicate actual coolant temperature. If not, this may also indicate a problem with the ignition switch or wiring.
This whole process will "waste" about 2-3 seconds of your precious time every time you start the engine, but may prove invaluable in helping to diagnose the no-start condition.

Thank you very much, as I can see you know what are you talking about. Thank you again.