Helping a friend with his water problems. Pressure was low and sometimes drops to nil. Submerged pump puts out strong flow. Changed the switch to a new 30-50. No problem with power supply. Also installed new pressure guage. The problem is this --- The cut in and cut off pressure varies with no adjustment to the switch. Tried another new switch and the problem persists. Would incorrect charge pressure or a bad bladder cause this problem?? The tank is a Welltrol, approx. 12 years old.. Thanks for any response.