2002 Dodge Grand Caravan Sport: Ignition Starter Problem

About half the time, when I try to start my van, the key
won't turn in the ignition. There is a crank, there is fuel injection,
there is also engine turnover. But for some reason the "magic" does
not occur.

I tried both keys that came with the van
(bought new), as well as a spare. Interior lights, head lights will
work. Equipment lights, clock etc do display; everything works as it
should pre-ignition and post-ignition.

I checked with a mechanic - and he stated it might be a security
problem with the "keys." However, after checking with Chrysler
Support, there is no malfunction with the security light/l.e.d. and the
key is recognized by the Engine on-board computer. Remote unlock/lock
works and the panic button also works.

Van has 3.3 V-6 engine, 150k.
Checked for recalls was negative; did not locate any service bulletins.

Any suggestions?