Cleaning your pH probe with a strong acid or base or any kind of harsh cleaning solutions is a bad idea!
What causes pH probes to slow down and fail is the buildup of scale on the bulb.
Mix up a dilute citric acid solution and soak the probe overnight in that. Use 1/2 tsp of citric acid crystals dissolved in 50mL of DI water. Citric acid crystals are available at your LHBS.
When I notice a pH probe taking longer than usual to settle on a value I will store the probe in a 4 pH buffer overnight. If that doesn't work I will do a citric acid soak.
Also, do not "scrub" the bulb on your pH probe with anything even slightly abrasive. Use a fine water jet, like the one delivered by a squeeze bottle, to do the mechanical cleaning of the bulb. Then pat the bulb dry with a soft cloth or soft paper towel.
If your pH meter/probe has a cap to cover the probe end with, keep the cap filled with KCl probe storage solution. If you store you probe in a beaker or flask you may prefer to store it in a 7 pH buffer solution. A 7 pH buffer solution is almost as good as a KCl solution for storage. If you use a KCl solution in an open container you will find the liquid evaporates and makes a big crystaline mess.
It is VERY important not to let your pH probe dry out. If you have no storage solution keep it wet with plain water.
I use a Hach HQ40d pH/Conductivity/LDO portable meter
But I use it for more than just my hobbies.