had that problem on a 1480 once. ended up to be in the pto. emitting enough vibration to make belt go crazy. check out pto and gearbox. for bearing or gear looseness.
Checked gear box for loose gears and did'nt seem to be a problem. PTO bearing is in good shape. Wondering if it would help to put a pan under the belt to ride on. 1480 had pan, 1680 did'nt have pan, 2188 did,nt have pan. looked at a new 2388 and the pan is back. Think it will work.