Blazer- 2 wd , 4.3 l vortec , 6 cyl , 230k , auto tranny.

Symptoms; starts up first key turn, idles @ 600 rpm for less than 1 minute. Then rev's up by itself @ 100 rpm, continues to idle @ 700 rpm for less than 20 seconds. Finally shuts off as if you had turned it via the key. Does this in every gear.

Work done:
- spark plugs
- spark plug wires
- distributor rotor / cap
- new alternator
- vaccum lines
- new battery
- air filter (throttle body lubed)
- vss (to trans)

Does this in every gear. Drives/rev's normal, no backfires. Bought used; do not know previous history. Driven about 180 miles, since purchased. No codes are displayed. Any help is wanted and appreciated. Please and thank you.