Does anyone manufacture an alternative Linn Sondek belt pulley ?

The issue here is simple, the OP is setting the table up himself and clearly something is going wrong, leading to my post above.

I've had my LP 12 for 22 years and I've never experienced anything as described by the OP. If the motor is sitting wrong, the motor adjustment screws are out of whack. Belt might be worn out too. I suggest it's time for a tuneup by a person who knows how to set up and optimize an LP12. 

Cakyol,
I’ve set up countless LP12s as a Linn dealer over the last 30 years.  I suspect the problem is you’re trying to get the pulley to be perfectly vertical on its axis, which is NOT correct!  The pulley and it’s shaft ( unless you have a Radikal PS and motor) is designed to be at a slightly tilted angle, tilting out away from the center of the spindle or center of the table.  This is quite different from most other brands and I’ll wager that’s where your problem is.  They do this tilt so you can use the two silver screws that stick up to gently tilt the motor a bit more or less which causes the belt to move up and down on the pulley.  Because the pulley is “fatter” in its middle the speed of the table will increase when the belt is riding on the fat part of the pulley.  Thus it will decrease the speed as the belt moves up or down off the thicker part of the pulley.   If you don’t have the two adjustment screws, you need to get them or the motor will vibrate or wobble a bit.    Unlike other suggestions so far, do NOT use a talc or conditioner on a Linn belt!!!  If your belt is more than 6 or so years old, just buy a new one!  If you have no way to show the speed of the platter or are unsure about what you’re doing, then take your table to a knowledgeable Linn dealer or at least to someone who has worked on LP12s before.  Now if you have a Radikal motor and power supply, it’s motor does NOT tilt and it self adjusts its own speed, so no adjustment is necessary.  One more thing to add:  normally the belt does ride near or on the fat part or center of the pulley.

I thought Linn recommended talc on the belt.  I always wondered because it seemed it would enhance slippage, but I never had a problem with it.  

I never saw anyone from Linn put talc on a belt!   Perhaps you’re thinking of VPI.  They often recommend that.