cr8tor,
I ran into the same situation with my T3P3 Kossel XL for a couple weeks (2 issues resolved a week a part)
- Pulley not tight enough (slipping)
- Pulley not on correct position of shaft (too far in or out)
Once I corrected those build / mechanical issues, flawless with near consistent 0.017 std dev between calibration runs; no bed mesh compensation needed.