The problem is that the speed is set by the total resistance in the section of the circuit for whichever speed you have selected. So dirty (= high resistance) contacts on the R8 pot, and/or the switch which is in series with R8, and/or R9 which is also in series can all cause the wrong speed at 33rpm. Similarly R7, the switch and R9 control 45rpm.
Cleaning the switch is straightforward, since you just need to apply some contact cleaner and operate the switch 20 - 30 times. For R7, R8 and R9, the technique is to apply cleaner and then move the pot through its entire range so that the moving contact in the pot wipes across the whole of the resistance element inside the pot. After you do this, just set all three, R7, R8, and R9 to the middle of their range and adjust 33rpm with R8 and 45rpm with R7. Leave R9 in the middle, and you will then have +/- 3% on 33 and 45 afterwards.
Good thought from the others on frequency/pulley. Denmark is 50Hz, so it shouldn't be a 60Hz motor since that would be too slow, and likewise a 60Hz pulley running on a 50Hz supply should also be slow.
You should be able to get Deoxit products from a hifi shop. They are also available on eBay if you use that.
And remember that there is no such thing as a stupid question. You asked because you didn't know, not because you are unintelligent