I have had 6 off SL150 (the motor unit version) turntables over the years and still have 2.
All have at some time required cleaning of the 3 way speed/power switch. I have not had to clean the pots on any.
To gain access the base has to be removed (power disconnected!), always remove the platter before doing this, no matter how tight you think it is, you can be assured it will fall off when you turn the deck upside down. The switch can be carefully dismantled and the contacts cleaned, I use a fibre pen cleaner for this. The contacts are open and will oxidise with time, the speed switch contacts carry very little current and any contamination will give problems.
good luck, well worth the time.