Pump in Spasms

[Poeme]

Appealing for help figuring this out.
I came home to the boat 2 days ago to find the analog amp and voltage dials on the electric panel in large rhythmic spasms...something going on/off/on/off, less than a second apart. Turning off the breaker to the domestic fresh water pump, the needles went back to normal. All faucets were closed shut! Yet the pump must have been cycling rapidly like this for the 48 hours or so I was gone...discovered the motor too hot to touch.
It was an 8 yr old Jabsco variable speed pump...so I convinced myself its electronics had gone bad internally. So, off for a new pump, Jabsco again, non-variable this time, very nice one...and took some pride in a clean installation.
On start-up I looked forward to the usual purging of air from the lines with faucets open followed by a steady stream...but no...
Water did come out all faucets, but then this pump went into convulsions too, very fast, very regular on/off spasms, identical to the old pump...brrrp, brrrp, brrrp...as if the pressure cutoff switch would immediately shut off the pump as it tried to come on, in rapid succession.
Tanks are full, water does flow, and this is nothing like "hammer-lock". Don't think electric short bc the breaker doesn't trip.
If there were a blockage in the lines, why would water still flow so well to the faucets? And why are the spasms so mathematically rhythmic?
A faulty breaker? Do breakers go bad? Remember with the old pump, this was going on with the faucets OFF!
How should I test for this?
Very grateful for any ideas...thank you Cruisers Forum!

[Telesail]

Have you checked pressure on bladder tanks and that the inline filters are not clogged with sand etc? The solution may be to pump up the tank reservoir since it could be cutting in and out as it goes through the activation pressure - sorry if terminology is not exactly correct but hopefully you know what I mean.

[donradcliffe]

check pressure water lines for a leak.

[Poeme]

Think I've narrowed things down to a blockage somewhere between the pump and the outlets, on the cold water side. Hot water flows well...the spasms only occur now when a cold water faucet is opened. I've got a job ahead for sure, either trying to blow or ream out old copper tubing. Please wish me luck, thanks for your thoughts...

[Madehn]

Most likely this is caused by a flooded pressure tank. Drain tank chech diaphragm in tank then pump air in tank to appropriate pressure. Your water heater tank probably has enough air in it to keep the pump from cycling on the hot water side. Your cold water accumulator tank is likely the issue.