I have a customer that is complaining that her Pro 5000 t'stat we put on a month ago worked great at first. It ran for 20-30 min & went off for 20-30 min. Yesterday it started running for 20 min. or so then off for 5 min then back on. A/c is working great, refrigerant is on the level. Replaced t'stat with a Pro 3000 & have same trouble. Any advice?