Pump troubleshooting

[PoolDoc]

Check your voltage at the pump, with the motor running. If the voltage is low -- I'm betting it is -- that's a motor killer.

However, if you check the extension cord, and it's anything smaller than 12-3 . . . you'll need to replace the cord.

[Poconos]

If you have access to a clamp-on ampmeter then measure the actual current the motor draws. The current will change with motor loading. That along with the voltage at the motor like Ben suggested will tell you the true facts of your pumps operation. You are near the breakers trip point, according to the motor plate, and that's not so good.
Al

[BigDave]

The change over time may be attributable to normal wear and tear if you've been using the same extension cable. Cable insulation can breakdown over time due to exposure and heat. Heat increases resistance in cable. Resistance lowers voltage. AC motors spin at a fixed speed and will use as much power as necessary to keep that speed. so... as cable heats up, resistance goes up, voltage goes down, as voltage goes down, current (Amps) goes up because Volts * Amps = Watts (Power) and power is fixed.

As Ben points out, low voltage will kill your motor this is because the windings are sized for a certain amount of current when current goes up the windings overheat and ... time for a new motor. Ben's right that a larger extension cord may help, 12/3 may not be enough depending on length (resistance is also a function of length). Be aware, 10 guage cords are very expensive.

[moriskod]

Did you have the motor capacitor checked?? I would assume that was done with the other work.

Interesting problem!