help loss of power 95 max se

[95maxsewv]

ok. i have had this car for about a month now. it has 99k on it. the 4th day i had the car the check engine light came on. i checked the codes. it came up with a engine coolant sensor error. these are only about 25-30 dollars so i can replace this easily. i reset the ecu and drove almost 3 weeks before the light came back on. this time it threw two codes. the engine coolant sensor and the rear heated o2 sensor. the car is getting about 24 mpg. the car also seems to have more power before it warms up. it still idles really smooth and runs smoothly, but it doesnt seem to have the power it did when i first got it. there also seem sto be kind of dead spots in the acceleration at different rpms. i changed the spark plugs in the car the day i brought it home. it had ngk iridiums in it and i put regular ngk spark plugs in it because i didnt do my research first. could this be part of the problem? i have replaced the air filter, pcv valve, it has 5w 30 sythetic in it. i use premium gas. it still runs great it just doesnt seem to have the power that it used to and that 190 hp should produce.