What you have is not caused by ground lift on the lower wing. It sounds like it's far too consistent for that. It may be that you've got a loose wheel bearing that resonates at a certain speed. You've probably seen this from time to time. Try putting a bunch of different wheels on a wire axle and give 'em a spin. At least a few with the "right" size wire axle will sort of flutter at some rotational speeds. If you left wheel is doing that then it'll add a LOT of drag on that side of the model. Or it just may be that your landing gear struts are just flexing for some reason at around that speed? Without seeing what it's like it's hard to say

Another possible cause could be that aileron trim. For a lot of reasons related to stall charactaristics you should strighten up that upper wing and remove the aileron trim on the lower. Nothing good can come from such a fix. It may just be that this is part of your problem. As the tail settles the down aileron on the right side could be generating extra lift thanks to the pinching of the air compared to the left wing where the up aileron leaves a more open gap. That could be enough to drive the left wheel down and produce more drag on a grass strip.

So check the wheels first. See if you can get the left one to flutter. If you can then try a bit of oil on the axle and try again. But if it's noticably wobbly where you can TILT the tread more than about 1/16 to 1/8 side to side then you should assume that it could flutter on you and fix it with a better fitting bushing from some brass tubing or just get new, better fitted wheels. If you got it to flutter and fixed it try it again. If it still does it anyway then it could well be due to your trim scheme and it's time to get serious about that warp.