Problem with jQuery and multiple ajax calls

I've noticed an off and on problem with ColdFusion/jQuery and multiple Ajax requests. The set up is fairly straight forward. I'm using CF 8 with the updater, and the latest stable version of jQuery. I have a function that is called when a combo box value is changed. When the combo box value is changed, two ajax requests are called to update other parts of the page.

If I run each of those requests manually, then there is no problem. However, I'll notice that they do not work about 30% of the time. Either ajax request can fail, so it's not specific to one call. I'm running Firebug and the errors that are reported from CF are unusual. Basically, CF thinks that a application variable is undefined. However, if I trap the failure and dump the application scope, it's always there. The actual application variable that is reported as undefined seems to be fairly random.

Has anyone else experienced any problems like this? If I take the same (ajax) request and run manually - it never fails. If I reload the page and try the same process again, it will work 70% of the time.

The requests look completely normal in Firebug, so I can't think of any reason why the calls would fail in one instance and not the other. And certainly not as random as it appears to be now.

Thanks!
Mike