Hi all, I'm looking for a suitable load testing tool for a web app.

I need to hit the site first with a login (username, password, then submit the form), then adjust a few select list boxes, insert some dates into some text boxes, and then see how long it takes to get data back and displayed on the screen (via Ajax/JQuery I think). I'd like to test this for up to 500 concurrent users if feasible.

I already have a WATIN web crawler app I wrote which would be ideal, but it requires an instance of IE to be open for each thread, so that's not suitable as performance on the 'runner' machine becomes a problem.

So far I've tried:
-loadimpact.com, good but some bug stops the tests from running, I reported it.
-loadstorm.com, apparently not able to replicate form stuff.
-Apache JMeter, apparently not able to replicate form stuff too.
-FWPTT, simple enough, but crashes.

Can anyone suggest a tool, or a usage of the aformentioned ones to achieve this? TIA.