Possible ignition switch problem?

We have replaced starter on a 2003 MB C240 with just 50K . Problem is when turning key the old and
now the new starter solenoid stays engaged turning the engine not allowing it to fire. Turning key off
wait a second try again and normally it will start properly with no effort. Once in a while it stays
engaged again but after a second try it typically starts right up. The old starter worked fine until we had
this problem with the starter solenoid staying engaged. Changed out the starter...not a fun job as you
know having to drop the entire exhaust system to change the starter. Then for several weeks all was
fine and then the same problem started again with this new starter. So it appears that the original
starter and now the new starter are not the problem. Best as I can see it may be the ignition switch but
have no way of knowing. No indication of any problem with the switch. Not hard to turn, no catching or
any physical sign of a problem. Key has only 2 keys hanging on it so can't say the switch is overloaded
with a heavy set of keys to create some mechanical issue. No error codes on the dash or other issues
have cropped up. Hate to think we spent $400 for a new starter for nothing...