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Slipping at constant speed

2003 Mercury Sable Wagon, 120,000 miles, mostly highway. While maintaining a constant highway speed, every once in a while the transmission feels like it slips. This only lasts for a second or two. Transmission fluid just changes at 100,000 miles/6 mos ago. Does anyone have any suggestions as to what I should start looking for to diagnose?

Was that the first change? the one at 100,000 miles?

What’s the tachometer doing when this happens? Does it go up? If not, it may be a slight engine misfire and not a transmission problem. Did you change your plugs at 100K miles?

Sorry, first change was at 50,000

I did not notice any change, will have to pay more attention. Yes, plugs were changed at 100,000 miles