Random appearing distortion while playing LPs


Hi
I have a problem.While playing LPs,i have a random appearing distortion that sounds as if the radio drifted off the station.The distortion appears not on every LP (today i encountered it on 3 out of 9 records i played)and lasts from several seconds to half a minute and on several occasions it dissapeared when i approached turntable.
What can cause this?
Turntable :Michell Gyro SEmkII,Rega rb 600 arm,Grado Reference 4mv cart.The phono stage is Pentagon (solid state ,as everything in my system)
overhang

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