Garrard turntables
I've got a Garrard model 50 that is practically identical to that AT 6. They are VERY nice turntables indeed. One of the things that will probably need servicing is the idiler tire which resides underneath the turntable platter. It's a rubber tire that goes between the motor shaft and the platter. Over time, the rubber gets hard and crusty, causing the turntable to run offspeed. I bought one for my model 50 at a local electronics shop about 10 years ago. They are still avaliable. MCM electronics will probably have them too. The first sign of idiler wheel trouble is that the turntable platter will stop when changer functions attempt to operate. Rumble and background noise will also increase as well.
While you're in there, lubricate the changer gears with lithium grease or other equivalent material. Also, lubricate the tripping mechanism. If this gets stiff, when the tone arm gets towards the end of the record, it may start skipping, as the tripping mechanism offers resistance to the tone arm as it travels across the record.
My turntable was my Father's, and he installed a Pickering cartridge in it. I further upgraded it to a Stanton that one of my friends got from the radio station he works at. These turntables don't like extremely light stylus pressures, so the DJ grade one, which is designed to run about 3-5 grams, does much better than some of the ultra-performance ones designed to operate on 1-2 grams.