ignition problems

[BLE 'BURBAN]

I've been experiencing this problem on and off since I first got my suburban
in Sept of last yearand BEFORE i started to do any mods on it.

What is happening is every once in a while, when I shut the engine off and immediatly turn the key all the way back to the accessory "ON" position, the engine will continue to run. Voltage gauge, temp gauge, fuel gauge and the oil gauge all shut off but the engine is still running. I can rev it up and let it idle. But it won't die until I shut off the ignition switch to pull the key.

Could this be caused by a faulty ignition switch or should I look further?

This is a T.B.I. engine not carbonated so I know it's not "engine run on" like the old carbonated engines did/do when the timing/choke/idle speed aren't set right.

Any idea's?