'88 Plymoth Voyager hesitates

Hello everyone... Recently my van started to hesitate while accelerating, usually within 15 minutes of driving it. The tachometer goes up and down. It usually goes back to normal if I let go of the gas
pedal (which doesn't work while it hesitates, I step on it, but nothing happens) and let it slow down to about 35 MPH. So far, I changed the spark plugs & wires, fuel filter, air filter. A friend told me it could be the oxygen sensor, what do you guys think?!
Thanks in advance for your responses... :-)
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