Hayward Pump Acting oddly

[Griffon1967]

Good afternoon,
I was just at a friends helping him reconnect a 110VAC Hayward Pump back to his control switch. Everything looks fine and apparently the pump is about 5 yrs old. As stated, its wired as 110VAC.
The strange thing is when the pump is primed, it starts normally and pushes the water through the system. After about 20 seconds, when the pump is building a good pressure prime, the pump oddly starts cycling on and off as if someone is throwing a switch on and off. About every second.
When the pump is opened and some of the pressure is released, and the pump is then restarted, it starts off fine again. As soon as the pressure comes us, this wierd behavious begins again.
I've had hayward pumps for the past 20 years and never seen anything like this before (although my pumps are kept well and I've only ever had two).
Does anyone have any suggestions I can offer my friend before he hands over $500 for a new pump.

Thanks to all in advance,

 

[Bama Rambler]

What is the pressure gauge reading just before the pump starts cycling?
Does it appear to be running as fast as it should be?

 

[Griffon1967]

Hi there and thanks for the response.
I'm not at the pool and didn't take note of the gauge (I'm not even sure it was functioning properly).
Anyway, I was just wondering if this was something someone had seen before so I could offer this friend some direction. In all honesty, I don't want to take ownership of his issue (I've got my own pool to think about) but was simply wondering if there was a simple explanation.
If I end up back there again, I'll take note.

Once again, thanks for taking the time to read and offer a response.

Kind regards.