'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?
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