1997 Chevy Blazer large drop in gas mileage

last year i replaced inigion cap rotor new cables coil plugs gas filter..this year i replaced 4 o2 sensors....in the last 2 months i noticed a drop from 15 to 17normal winter milage to 9 to 10 miles per gallon ..this year is a mild winter and do not let ingine idle long for warmups...had this vehicle about 7 yrs...the only thing i notice is when the weather is damp or rainy i feel a rapid bucking which goes away as soon as i feed it gas..i really try keep this car up to par....any ideas relcome....