'95 Neon loses power on highway

I recently bought a 1995 Neon, auto tranny, that idles great and has no
problem around 60km/hour (35mph) but after being on the highway for 10+
minutes at 100km/h (60mph) it feels like it's running out of gas. Tank is
half full.

When this happens, if I stomp on the pedal the transmission kicks down a
gear and it will accelerate normally. When I lift my foot to maintain my
speed it starts to lose power again. Once I get off the highway the car
seems to run OK.

Does this sound like an O2 sensor? I'm getting the O2 sensor code. Anything
else it could be?