Q: Not turning over spinning - 1980 Mercedes-Benz 240D

The starter on my car doesn’t always crank. But it happens on about every 22nd time. I can hear the starter spinning fast when this happens. Once in awhile it will crank the engine for a fraction of a second then spin the starter very fast. The car has a 3 month old genuine Mercedes Benz steering lock and ignition switch. But this problem started before I installed the new steering lock and ignition switch. The battery is 2 years old and checks out good. I noticed when the weather is cooler out and rainy the starter spinning happens more often. Could the problem be a bad solenoid, bad starter gear, bad starter or something else? Do you have and ideas? Thanks.