Treadmill operating on undersized AC wiring and/or is drawing high current.
Verify AC feed into the building's circuit breaker is correct and connected to the proper transformer tap.
The AC wiring must be sized to handle 20 amps of current over the loop length of the AC wiring. The voltage drop across the AC input wiring is a result of the length of the wire run multiplied by the amount of current being demanded by the treadmill. The longer the run and the higher the current the larger the wire must be. Verify the unit is connected to an isolated and dedicated 20 amp circuit.