1999 Dodge Neon 99 Dodge Neon

The car will lose power after about 20 min on freeway then u have 2 let it sit and it will start up and run ok now it is doing it in the city driving changed the cat and problem is still there and ti has great fuel pressure it is at 43 to 45 psi check engine is on and has been on 4 a while so went 2 autozone and they scanned it they sead it could be o2 or mass sensor could that cut the car off and there is a lean or a rich code hope u can help me with this ty mark cassidy