My keys are not working correctly. Every so often, one or the other won't lock / unlock the car for a few attempts.

Every so often, the ignition accepts the key, things light up, but when turned to start, the engine does nothing. If I try the other key, it usually works.

Today, neither worked for a few attempts. Then on the last try, I put one of the keys in, turned it, and left it turned. After a few seconds, the car started up. I started the car a few more times today no problem.

I've changed batteries, with no change in symptoms.

What can I check, clean, or generally fiddle with to help solve this problem?

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport