I've had a couple of Blagdon pumps running my pond filter (24/7 all year round) and my stream (on and off as required) for coming up to 4 years now and haven't had any problems with them at all, with the only maintenance being occasional cleaning - about every 3 months on average but more often in summer.
I can't imagine what's causing yours to lose power except to say that in the summer months when the oxygenating plants are quite rampant I do have to keep an eye on the pump filter heads to make sure the plants aren't blocking them. If they do the loss of pump volume is quite noticeable, though I doubt that would lead to them burning out.
As for it being better not to use a pump (presumably for filtration), I know that ponds are supposed to find a natural balance after a while but pumped filtration is the only way I can keep my water crystal clear and that's how I (and our visiting Kingfishers
) like it
Jeff
(Schedule 1 Licence holder for Kingfishers and Barn Owls)