OP
1)It is either a "wobbly wheel" meaning it is out of true-
Fix it by
1)Have someone hold the rear wheel off the ground
2) you spin-by hand-the wheel-slowly
3)Notice where it touches-or nearly touches(might not touch with no weight)
4)Find EXACTLY as possible where it touches MOST
5)make a mark on rim with tire touches MOST
6)Make 2 marks about 10" in both directions from touch point on the rim
7)Get a spoke wrench -or a tiny adjustable wrench or pliers
8)Pay attention-you are going to ADJUST-TWIST 4 spokes-maybe 6
9)They will all be spokes that will PULL the rim AWAY from the frame
10)Look CAREFULLY at where the spokes GO- half of them will pull the rim AWAY from the frame on the touching side-the other half-would make it worse
11) The 4 or 6 spokes will be 2 or 3 in either direction from where it touches MOST
12) Turn the 4 spoke NIPPLES about 1/2 turn- CLOCKWISE looking from ABOVE
13)if improved-do it again-but turn the spokes closest to the touch point-MORE than the ones farther away

Oh- the wobbly wheel OR-sometimes even vertical dropouts will allow a wheel to be placed in somewhat cockeyed usually you would have discovered this a long time back
Hope the above helps-
Usually this is caused by a cockeyed wheel-or even a tire cockeyed on rim
Just a PICTURE would help-no video-just a couple of pictures
Luck
Charlie