My van will not crank when I turn the ignition key.The engine will not turn over on 1999 Chevrolet Express 2500

If I jump the starter solenoid with the ignition key in the on position it start up. After itnruns for a while it will crank with the ignition key, but if I let it set for a while I have to jump the starter solenoid again

Asked by for the 1999 Chevrolet Express 2500
have anti theft codes scanned to see if you have any faults with alarm to start with. you may have a key and or skim issue if this van have that system
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possible ignition switch