Best guess with the information provided is restricted flow of the catalytic convertors.At higher speed the rpm's stay consistant. If the cats are not allowing the exhuast to push thru unrestricted the engine will know its labored but not why.
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If it is full of oil, then check the coolant level sensor. I may be low of coolant, or the sensor on the side if the radiator tank may be dirty. Just remove the sensor and clean well and replace.
If the engine has a lot of sludge, the drain holes from the head back to the crankcase may be partially stopped up. The valve cover may be filling up with oil at higher speeds, but then it drains back as you slow down and seems full when you stop and check it. Good luck
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