Hello Dave Hollen,
It's expected behavior, because IE doesn't handle dom elements so good and you're adding too many items.
I recommend using RadComboBox instead and load the items on demand as it's demonstrated
here.
Greetings,
Yana
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Explore the
Telerik Public Issue Tracking
system and vote to affect the priority of the items.