Although Check Valves are common to behavior of this type. We would suggest to take more of a diagnostic approach by performing a Static Decay Test. This may give a better understanding on the next step to take in replacing another component such as a pump seal. Ensure that your pump seal solvent wash is suitable and working well.
What solvents are you using and how large are the pressure fluctuations? What flow rate are you running and what is the system pressure? There is no question that faulty check valves are the number one cause of pressure fluctuations. Having already replaced the check valves with no effect, the resolution is either much more simple or much more complicated. The simple answers would all involve some variation on the fact that solvent is not reaching the pump. This could be due to a leak somewhere between the solvent reservoir and the solvent select valve. A faulty degasser chamber could prevent solvent from reaching the inlet of the pump. One of the solvent lines may have lifted itself out of the solvent or the chosen method selects a solvent line that is not properly primed. If none of the simple answers resolve the problem, then the suggestions made by Liz are the next step. Perform the various pump diagnostics and attempt to determine if there is some type of leak in the pump heads that is not, obviously, visible.