The two most common reasons of a fluctuating UV-signal are the lamp and the flow cell. Make sure that the lamp did not exceed 2000 operating hours. If this is not the case the flow cell could be dirty or there could be air trapped in the flow cell. Therefore, clean the flow cell according to  the manual or flush the flow cell with degassed solvent at higher flow rates. To prevent air bubble formation in the flow cell a back pressure should be applied behind the flow cell. Back pressure can be generated by either tubing with a smaller ID or by installing a back pressure regulator.
We suggest a back pressure regulator, which is adjustable between 1 - 20 bar (A70087). Most of the times 1 or 2 bar of backpressure is sufficient for a stable UV-signal. Keep in mind not to place the back pressure regulator behind the pH electrode because it is pressure sensitive (pressure limit 5 bar).