Cold startup issues power and mpg issues

[3.4King]

Started my car this morning it was chilly has been all night and haven't started it in 2 days she made a roaring looping sound and had no power surged when I gave it gas. Now it surges sometimes when taking off even after it's warmed up and when u try to get on it or for up a hill it hesitates bad like you are hitting a pocket of air. Recently changed oil plugs wires and both o2 sensors and I also only get about 16 mpg car is a 95 3.4 camaro with a supposed rebuilt motor not sure of actual mileage. What can I do? And I drive sensible