There are lots of problems here. One if the extension cord. I can't imagine any contractor hooking anything up permanently that way. Hopefully, it's just temporary.
Second is, what brand pumps are you using? If you don't buy a quality pump, don't expect it to last long with this type of duty.
Third, I would try my best to find out how many gallons per minute are running into this pit so I could try to find a pump that will do about the same volume so it doesn't cycle itself to death. If the pit is big enough, you can tether the cord out longer so the pump goes on for a while then stays off for a decent amount of time to give the pump a rest. If the pit is small and a lot of water is coming in, I would consider making it larger.