Great,
If it’s lump lump then it’s probably V Wheels, maybe one of them has fractured or grown a dent from being over-tensioned, could also be the bearings, maybe one ingested some crud?
When I had my V Wheels over-tight to try to get rid of spindle vibrations the machine would have a really noticeable lump lump in X for the first few minutes of operation thanks the the dents that had formed in the V Wheels whilst the machine was stationary, there’s some evidence of those here
Generally after a few minutes of moving the lumps had flattened out, it was a sign of me abusing the V Wheels though and remember that backlash goes up with V Wheel tension as the extra rolling friction has to be countered by belt extension.
HTH