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Your engine might be lugging at those RPM ranges.
It's very odd, because most engines don't lug anymore over 1500RPM. But it is still possible, especially when the area is hilly (going uphill).
Basically, the oil pump is rotating so slow, that there's not enough oil pressure to lubricate and cool all the parts.
Usually first the valves would start ticking, then some bearings will run dry.
It's really bad for your engine.
Also, according to the statistics, you were doing below 1500RPM (1439), not 1700RPM. You may think the load on an engine is low, however when the pistons are going too slow, and it's still pushing a 3 ton vehicle, there will be more metal to metal contact.
What would help? A short rev up to 2k or 3k RPM, just to get the oil flowing again, cooling the parts that need oil cooling, then you can cruise again for a few minutes at low load.
My Honda Rebel 250 did the same when I reached below 2000RPM. This was a high rev engine feeling best at 3K and 6k RPM.
On that machine it was easy to hear the valves ticking, and pulling in the clutch, and revving up the engine stopped the ticking sound.
Since then on my small Honda, I never went below 2250RPM, and never had any issues with it, no matter how I abused that engine!
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