Steps to repro:
1) Run this test: https://drive.google.com/file/d/0B-p_FKXYznm5cGJJbUlab09hYlE/edit?usp=sharing

Expected: Test Studio detects we have an AJAX request and handles things for us perfectly.
Actually: The test fails with this exception: Index was out of range

This test fails unless you manually add a DelayExecution step before the 'Select item" step. This is because the items of the combobox are loaded on demand via an Ajax request. And if you don't wait a bit - the list contains 0 items.