ignition switch probably bad.After starting,turn the ignition switch back just enough so your not engaging starter.If it stays running replace switch.
vacuum leak, bad idle control sensor
Idle is set by the computer.
Could be the throttle position sensor
Auto-choke is stuck open
I would start by changing the thermostat. They go out, often, or have a tenacy to stick, and are cheap.
1500 at first until it kicks down to 850 rpm
You don't, it is done by the ECM (Engine Control Module).
Possibly the IAC ( Idle Air Control) vavle on the throttle body, or egr vavle stuck open..........would check or replace IAC first, usually not expensive.
one of ur sensers are bad. i would look at the idle air sensor or coolant tempuratur sensor
I would start with the Fuel Pump.
I have 91 chevy k1500, It does this, Sometimes it will idle a few minutes, then jump to high idle and check engine light comes on, And sometimes its as soon as i start the engine... I was told it was a vacum leak, But im no mechanic
check the soleinoid