The most common causes of a rhythmic or wavy baseline are related to the pumping system. Typically, your pump will have two pistons and seals. If one is more worn than the other, this can cause flow and pressure variations in a very rhythmic pattern, which will also be seen by the detector. You can confirm a pump seal problem by varying the flow rate; the frequency (in time) of the baseline fluctuations should increase/decrease proportionally to an increase/decrease in the flow rate if the pump seals are worn. The solution is to replace the pump seals.
If you have a multiple pump system (binary, ternary, or quaternary), the problem may also be a function of insufficient mixing or of a malfunctioning proportioning valve.
Finally, another possibility may be the detector itself, though fluctuations would not tend to be rhythmic. Again, varying the flow-rate as mentioned above can confirm pumps are the issue, but if adjustment of flow does not have an effect on the baseline fluctuation, the detector itself (e.g. aging lamp if UV) could be the cause.