Had this same problem on a customer's van recently.  I thought it was the
ignition switch until I hooked me remote starter switch up to the starter to
crank it from the rear of the van.  The starter stuck on again.  So the
solenoid must be sticking (maybe this cold weather is a factor?).  Rebuilt
starter would be a good idea unless you like to tinker.  I have yet to have a
cleaned up solenoid last for more than a year.

Thanks,
Ken Wilford
Van-Again
John 3:16
http://www.vanagain.com
Office: (856)-765-1583
Fax: (856)-327-2242