Hey!
I know what you're talking about! There I am, chugging along quite happily and, "pass your message" comes over the speaker....
Beh, Urrrrr, Ehhhhhhhrrrr, Duurrrrrrr! Very quickly, the limits of my mental capacity are reached. I seem to have some sort of mental 'block' over RT.
Luckily, there are a number of things that have worked out for me:
There's a pc based interactive course (Oxford aviation training), but it's quite expensive and the material is limited to a couple of x-country exercises. Once you've done them, you've done them. Also, the license only alows you to install the software twice after which time you have to buy another license so pick your PC's carefully! There's a CD recording (Asco airspeak) which you can listen to in the car. Again, it covers the syllabus competently but with limited examples. Also, there are no track breaks (on the version I bought, anyway) which can make finding specific examples time consuming. I got round this by taping to mini-disc and inserting my own track breaks. Trevor Thom book 7 elaborates on CAP 413 as well.
You can't beat good old fashioned practise though. I try to listen to actual conversations on a scanner at my local A/P and MS flight simulator 2004 has some RT built in.
All the best!