I presume this is an isocratic method, but in case there is a proportioning valve in the flow path, such as with a Quaternary pump, bypass the gradient valve, or try premixing the solvents instead, or clean or replace the valve. Since this drifting is common to three LC systems it seems the cause is unlikely to be a hardware failure. Column bleeding can be the cause. If you think this is possible flush the column with strong solvent; also improve the sample clean-up and use a guard column. You might need to replace the guard and or analytical column.
Hope this helps !