Random appearing distortion while playing LPs

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)
I was thinking the same thing as Shardone.

do you clean your stylus and records before playing? That might solve this issue. FWIW I have had TT's in very dusty environments and found the same thing but if I clean the stylus and records before playing, I don't have this issue.
The clue that struck me is the possible/apparent interaction when you approach/leave the turntable.

This may be a variant of Grado hum, caused by its famously minimal shielding and susceptibility to electromagnetic fields. Your body may be affecting the surrounding fields in some way the cartridge is detecting. Are you in a location where static electricity has started to build up as colder/drier winter air approaches? That would tend to confirm the hypothesis.

Many people have issues with Grados on Rega tables due to proximity to the motor. Layering the underside of the platter with foil is sometimes said to help. Try that, can't hurt.
On my Michell the motor is outside on the other side of the shelf.The climate is getting dryer and colder.Should i zip Grado with Zerostat?
Missed the fact you're using a Gyro. Saw "Rega" and my brain turned a tonearm into a turntable. <;-)

I wouldn't zap a cartridge with a Zerostat. Don't know why, just doesn't feel like a safe thing to do.

Try wrapping little foil shield around the cart and headshell. Make sure it contacts the tonearm and that the arm itself is grounded. Easy to try and free. Or try another (non-Grado) cartridge if you have one handy. Might help you isolate the cause.