Faulty reset switch??
That would be my next thing to change as well. Doesn't matter how they test, they can flake out under load.
What tends to go is the motor, loom to head, PCB or reset switch. Seldom will you find a machine where something else causes this.
We had one come in the other day stuttering like that, I quoted for a loom. We changed that and it didn't fix it. We had to swallow a reset switch and then it was right.
buy a Dyson cordless or a DC50 for animals. If you are going to buy a new machine (which you don't need to) for animals, I'd probably go with a Sebo.