Fluctuations in readings in a UV-vis instrument are almost certainly due to a failing lamp.  A typical insturment which uses a xenon lamp is normally rated for only about 500 hrs, while a deuterium can range from 1,000-3,000 hrs.  However, they will sooner or later.  If you are not sure of the hours on the lamp, I would start by replacing it to eliminate any fluctuations. If changing the lamp doesn’t fix the problem, then you may need a service call.

Using the UV-Vis Calibration Kit will flag a problem such as this before erratic or poor data results from the bad lamp.

UV-Vis Calibration Kit Product Page