Sounds like intake impeller failure
Impellers do have a finite lifespan that can be shortened to about 30 seconds if you make the mistake of keeping the intake thru-hull closed and adding water to the bowl with a cup or the shower head. That is, if you're sure the intake seacock is open.Replacing it will be fun (NOT!), because to replace it in a Jabsco, you have to take the whole pump apart to get to it.Which model is your Jabsco?