Mike,
There are two possibilities and without test equipment difficult to prove what the fault is.
1) the motor is drawing excess current due to a fault and the thermal overload is doing its job in protecting it from further damage.
or
2) the thermal overload has become ultra sensitive through age and it tripping at a much lower current than it is set for ( 2.4amps as Kirk says)
Just after one of these trip episodes, is the motor hot to the touch, a little off cold or as cold as the nearby metalwork of the drill?
Provided the motor is not hot, then I would suggest turning the knob on the overload - usually clockwise for higher setting and you should see the scale move. Adjust it up to just over the 2.5 amp mark and see if things improve.
Look on the motor plate to see what the current draw should be. Normally the overload would be set at that figure or perhaps 10% up on that.
Try and avoid the temptation to turn it up all the way without first understanding what is happening. This is a protection device after all and setting too high could cost you a new motor as the windings become the weakest link.
If it turns out that the trip is weak/defective then spares are not likely to be available - contact your local ark breakers yard
A replacement NVR would be needed £30 at Toolsatan but quite a bit bigger and uglier than your current one.
If the motor is running hot after 15mins work, then that needs to be investigated and repaired/replaced.
More than happy to give continued support- please report back.
Bob