Thanks to your input above, I managed to figure it out.
Once you have performed your second rotation, you need to recalculate the screen translation vector to avoid the jump.
To do this :
1. get...

Well thanks for your input. However I've tested this sequence of instructions, but it doesn't address Langecrew's concern : "What I would like to try to do is to translate my model in a plane...

Hi

I was wondering : did you ever find the answer to this question? I've been thru the same steps as you have, all without success. My best guess would be something like :
...