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.