suburban not starting

ok here is my problem my vehicle starts then dies doesnt come on completely just doesnt stay on and the security light stays on , sometimes i do a rest with the key for 10 to 20 minutes then security light goes off and then suburban starts this has been goin on for 6 months usually in the mornings when it doesnt start sometimes it does but sometimes it doesny with security light on, what can be the problem and help is appreciated before i take it to dealer

What year is the truck? The VATS is different in different years, but generally speaking, if the SECUROTY lamp remains lit after the engine has started, that indicates that the system does not recognize the ignition key.

You may be operating in a non-secure mode. With some series of VATS (PASSKey and PASSKey-II), if an incorrect or intermittent signal is detected by the VATS after the engine is running, the VATS will allow the vehicle to be restarted, even with an existing failure. The SECURITY lamp will remain on to warn the driver that a VATS system fault is present. However, this also means that the car can be started w/o a correct key signal, and is therefore vulnerable to theft. An temporary/pending error code will also be set by the ECM/PCM in this mode.