It’s common for the FLC to fluctuate and the difference you describe is small. I have lambda FLC MGUS also for 5 years and the levels have bounced around between 25-90 with the K/L ratio fluctuating accordingly, sometimes normal, sometimes low. If the FLC keep increasing in an obvious upward trend, your Dr may recommend a bone marrow biopsy just to check the plasma cell percentage, hoping for under 10%. Most people with MGUS have no problems from it and remain stable. It just should be monitored periodically.