95 VX cruising issue - question

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I just bought a 95 VX a few weeks ago and just started driving in as a DD. I have a CEL (code 48 - LAF) coming up once I start driving the car for less than a mile at times and not until 6-7 miles at other times. When cruising at 50+ mph I can feel the engine delivering power then holding back, going back and forth like this ever so slightly when the CEL is NOT on. Once the CEL goes on this issue goes away, and the car cruises fine with no slight hesitation at all like it was. I am assuming this is a sign of the 02 sensor that has gone bad.

I have done a full tune up on the car with all the usual things noted on this website. Just did 39 mpg on the first fill up, but it was only 120 miles just to see what it was doing.

Would others diagnose this as a 02 sensor issue?

Thanks in advance for any help.