Could be the timer getting stuck intermittently or the wires going to the drain pump or drain pump has an intermittent problem........shopvac your drain hose line and clean out your drain pump you may find a coin trap that's clogged but the fact that your saying you can advance your timer and it works is suspicious and pointing towards the timer starting to get faulty or you have a bad/corroded connection at the timer or the pump. Also check your voltage supply going to the timer make sure your getting 120v and you have good and adequate wiring and breaker to use with whatever you may have connected to the same circuit if you have less then 120v you have a voltage drop or something going on in your electrical in the house
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