If you're talking about the sawtooth part right around 6k rpm, that's what a bumpy road looks like! Seriously. I zoomed in on your log in this screenshot. You can see that the MPH and RPM has the identical dips and spikes. That's a good thing, that means the clutch IS NOT slipping. If the clutch was slipping, you'd see the rpm's rise but the mph would stay the same or flatten a little even.
Basically what you're seeing is the suspension getting a little upset by something, most likely a dip in the road or possibly a little tire slip. It's easier to see this stuff in 4th gear when the rpm's and vehicle speeds should track evenly, assuming the trans is 1:1 in 4th.
Good question btw!
This is from YOUR log... Red trace is Engine RPM, Green trace is Vehicle Speed.