set values in Textboxes should not fail, you can write a small function, which try to set the value more times (this will increase the stability of the test).
In some cases the dialog is not completly ready so maybe wait till the element you want to use is enabled and visible.
You can try to use Thread.Sleep() to wait a short time, but better is you want for control depending things like visibility or item is enabled/disabled, and so on.
The comboxBoxes is a known problem, for fast machines
I use an own function to get this stable, its something like that
Predicate predicate = new Predicate(p=>p.Name == "item name");
Thread.Sleep(100); // be sure the drop down opens (for fast machines)
ListItem item = comboBox.Items.Where(predicate).First();
You can verify with comboBox.GetSelectedItemText(), that the item is selected (maybe more tries to set the value will be helpfull)
hope that helps