This phenomenon got me also shortly after I took delivery.  I 
really truly thought something was very wrong with my NEW tractor and took me a bit longer than I would like to admit to figure it out.  
I am on a ridge & have many trails and there was one section and at a specific point it would always almost stall out. Only happened at 
that one place, unloaded & going uphill - super weird right?  Turns out it was a deep rut that consistently bounced me a bit;  hardly at all to take notice, but 
just enough to unweight the switch, even though I was in low & going slow 
 
 
While not heavy, I am solid and will certainly not blow away in a wind, so this never even crossed my mind!  I got so frustrated after figuring it out and then having it happen in other places that I went the wire tie route -one quick snip and back to stock.