If you find your model engine is losing performance quality and you can be sure there is not an electrical problem, then the most likely cause is dirt and solidifying lubricants. The solution is to disassemble, clean and lubricate the engine. The process is a little messy and requires some degree of patience and care when working on the engine. However, it is not expensive, takes very little time and will make a big difference in many cases.
Remove the engine shell by turning the engine upside down and removing screws or gently prying open the plastic tabs, and set the shell aside.
Brush away all visible dirt, fibres, dust and debris with a quarter-inch flat brush using short, quick strokes.
Remove trapped debris with tweezers and toothpicks as necessary.
Apply a small amount of denatured alcohol to a clean cloth and gently wipe down the chassis, wheels and axles, and motor to remove dirt, grease and grime. Use a cotton swab or toothpick where necessary.
Pour a small amount of model engine lubricant into a clean bottle cap.
Dip a toothpick into the engine lubricant and apply small amounts of oil to the wheel bearings, axle bearings, gear shaft and motor bearings. Bearings are the points of contact that restrict motion of moving parts and are typically plastic or metal rings or housings holding a shaft or other part in place.
Test the engine on the track before putting the shell back on and relubricate as necessary.
It is a good idea to minimise how much you touch the engine since oils from your hands can create problems. You can create a foam cradle out of extruded foam insulation or a thick towel, though fibres from a towel can create problems. Cleaning and lubricating your engine is a good idea after every 10 hours or so of use.
Do not substitute household oils for engine lubricants. They are too thick, and can create problems.