there is a chance, if you had a new boiler fitted to an old system, that the problem is due to old dirt, sediment, grit, rust and corrosion from the old pipes and radiators.
In which case it is not the repairman's fault, but it might be the fault of the installer, if he did not insist on treating this (at your expense) before fitting the new boiler; or your fault, if he pointed it out and you refused to pay for rectification. Similarly if there was a pre-existing design fault causing air to get into the water, or a leak.
On a clean system, with a Magnaclean or similar to catch the dirt, and corrosion-inhibiting chemicals to prevent more occurring, a pump should last 10 years or more.
Boiler guarantees do not cover faults caused by dirt. They sell you a clean boiler, and they don't cause the dirt.
Of course, in your case it might be something else entirely.