broken drill?

Hello. I have a Bosch CSB500RLE, serial no. 0603 147 842 corded hammer drill. It is quite old but is a superb piece of kit. It has just begun to work intermittently. Sometimes it works, sometimes not, sometimes it starts if I shake it or turn it upside down. I have checked the brushes and they look OK, I also cleaned the commutator, though it looked OK. Any suggestions please? Could part of the armature/magnet have burned out - if so it's obviously not worth repairing?
Thank you in advance
John Lund