This is a common problem in wafer switch technology, potentiometers are a little different in terms of the cause but more of that later.
I have repaired many elderly pieces of equipment with the symptoms you describe and would advise the following.
1. Switch contacts can suffer oxidisation of the silver plating causing intermittent or high resistance operation. The only way to cure this is to remove the oxide layer either by multiple operations of the switch wafer or, more reliably, by chemically cleaning the switch. The crucial detail in cleaning the switch is access and the use of the correct cleaner. Please do not use oil based cleaners such as WD40, they are not great for this application. I would recommend an aerosol contact cleaner available from electronics stores. Choose one that is 'no residue' and does not contain a lubricant as that is not really needed on wafer switches and contaminates the contact substrate. You can use a lubricated switch cleaner if you really wish to but I personally found that it makes a real mess of the wafers and attracts dust. The cleaner should be sprayed onto the contacts, left for a few seconds, and then the switch operated several times. Repeat around 10 times to ensure a good cleaning action. Leave the unit unpowered during the process and wait at least 15 minutes after the last cleaning session before applying power. If this action does not remedy the situation you are dealing with either severely contaminated contacts or contacts that have lost their plating due to wear. In either case the best solution is replacement of the switch assembly. If this is not possible, the switch wafers should be disassembled and all contacts manually cleaned with IPA and paper. Paper is slightly abrasive and when soaked in IPA will rub oxide off the contacts. If the plating is still OK this should clean it, damaged plating cannot be repaired economically but may clean up enough to work for a while at least. I recommend that you do not use cotton 'buds' to clean wafer contacts as they tend to shred and leave strands of cotton all over the contacts.
2. Carbon potentiometers that are commonly used for variable ‘trim’ controls on equipment can suffer dirt ingress, oxidation of the wiper arm contact and deterioration of the carbon track. To remove dirt and oxidation build up, I recommend any cleaner that is designed for cleaning carbon potentiometers or a ‘no residue’ non-lubricated contact cleaner. IPA will also work well. It is possible to dismantle a potentiometer and clean the carbon track manually with a suitable cleaner and lint free cloth or paper. If cleaning does not solve the problem, the potentiometer should usually be replaced with a new part as carbon track damage is likely and not easily remedied. I have, in desperate cases, dismantled a potentiometer and re-aligned the wiper arm so that the contact traverses a different area of the carbon track . This must be done with care as poor alignment will rip the track up in no time at all.
3. Never apply switch cleaner to switch or potentiometer shafts, it washes out the lubricant and achieves nothing in terms of cleaning the contact surfaces. Switch cleaner should be applied carefull directly to the contact surface or carbon track. Some potentiometers have a hole in the cover that permits spraying of the cleaner onto the carbon track. The alternative is to remove the cover to gain access to the track and wiper contact.
All this is just my 2 cents worth and 30 odd years of experience refurbishing elderly kit. Others may know of alternative options.
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