The Nissan Path

Hi, I'm new here. Got the 2005 car 1.5 years ago with new transmission at around 105K and did bypass myself. Now it has 124K and here is my question:
while cruising I noticed a light slip, like overrev, at around 2K rpms at 50ish mph. The slip comes for a 1 sec and then dissappears. It jumps RPMs to 2.5K and back where it was. My foot is resting on peddal when it happens. Feels like slipping clutch in manual car. My coolants are clean, I checked just incase. I feel the car and I know it smth new. Any ideas

Ok I found it. I've been towing my boat 4.5K lbs for about 1500 miles last season and beginning of this month. Thin film of oil on dipstick shows that oil is clean only if you drain it into bigger container you see how burn it is. When oil is burn it looses its viscosity and ability to work under pressure and change gears. So I decided to start with changing the oil and that was it. Again, the symptoms were like slipping clutches.