1995 Chevy Blazer

WHEN CAR IS COLD IT BUCKS AND TRIES TO STALL.
THE RPM GAGE GOES NUTS AND THE MESSAGES ON
THE DASH GO ON AND OFF [CHECK BRAKES ETC] INCLUDING THE CHECK ENGINE LITE WHICH SOMETIMES STAYS ON FOR SEVERAL HOURS THEN GOES OFF. AFTER THE ENGINE IS FULLY WARMED UP
IT RUNS FAIRLY DECENT, WITH ONLY AN OCCAISIONAL MISS. GAS MILAGE IS HORRENDOUS,
ABOUT 9.5 MPG. I ALREADY CHANGED THE COOLANT
TEMPERTURE SENSOR TWICE AND THAT DID NOTHING. I HAD THIS ENGINE COMPLETLY REBUILT
WITH ALL NEW COMPONENTS. IT RAN PERFECT FOR ABOUT A YEAR WHEN THIS PROBLEM STARTED.