Have had this rig about a month, driving around town. Took it on its first several hour trip this weekend. Something seriously out of order. Help with diagnosis? Thank you.

We are in eastern Washington, lots of hills, temps yesterday in the high 70s.

Car loses power going up a hill, acts like it is flooded, speed backs to less than 30 mpg, more gas pedal it acts like it will stall; less gas pedal it will 'sip' and eventually work itself up to 45 mph on a grade usually passed easily at 60.

Car in neutral at idle chugs and chuff. At highway speed, car begins to lose power, down to 45 mph if rpms less than 3500; if you mash the gas pedal, rpms go up to 6000, car accelerates back up to 60, rpms drop back down to 3000....

Yeah we limped home like this 200 miles.

The gas mileage went from 21.3 down to 20.6 but I'm not sure how that works; is it the entire life of the car or just the last trip meter was set?

The car has only had one tank of gas run by me in mostly town miles; we purchased it in April. It supposedly has had its timing belt replaced. We did major service on purchase and the throttle bodies or whatever they are called were replaced, all 5 of them. The inspection they did stated the 02 sensor needs replaced for a $200 or so bill. Any ideas on diagnosis? I'm calling the shop that did the last work on it tomorrow.