I had an Linn LP12/Valhalla, I would suggest that you take it to a Linn Dealer/TT specialist because that Vahalla board has high Voltage and if you don't know what you're doing and don't know the general saftey rules for working on high voltage gear then I would not mess with it, one small slip of your hand and you can be seriously injured or even killed. It maybe something simple, Was the speed varying? because that's usually the first sign that the Valhalla is going bad otherwise it may be the switch itself or a bad connection. If its the Valhalla board this may set you back about $500-$700 for a new Valhalla but if the guy is good he may be able to repair the bad one with new caps for cheap because that's what usually the problem when the boards go bad due to having power supplied at all times even when the switch/power is off. I hope this helps.
Steve
Steve