The only fps diffence a higher tank pressure will cause, is if you are rapid firing, or at least quickly firing the gun immediately after a trigger release. In rapid fire conditions, a higher input pressure will cause your valve temperature to rise more than it will for a lower input pressure, causing higher velocity shots. When you let the valve sit a few seconds between shots, the temperature will cool and the velocity will go back down. Its a thing that retro valves are known for, so you need to be aware of it. Consistency when operating the trigger should yield consistent results.
If the tank regulator doesn't have a good flow rate or is inconsistent, it can cause velocity issues because the mag valve won't charge fast enough to reach full velocity on some shots and will reach velocity on others. The same thing can happen if the mag valve itself has oring issues or is sticking somewhere. Since you indicated the valve worked fine on another tank, then the original AA Appocolypse tank seems like it might be the culprit.
Except for the Automag in front, its usually the man behind the equipment that counts.