Nav 2009R - Perfomance issu for one user on RTC

Good morning,
Just before to start, i just want to excuse my english who are not very perfect.

My problem come from on one user navision account who are a very slow response and performance issu on RTC client since few months. We try a lot of solutions with and without our retailer. This last one don't have a sustainable solution.
We have almost 60 users on Nav and just one of them who have this problem. It's come from aleatory and often in the high activity's period. He is a key user with largest authorization on the product and he is a key users for us.
We try without success :

- re synchronize the profile some times that's work. - Delete the complete profile and created it again. It's work during few hours and the performance issue come back. - delete personification file without sucess. - uninstall navision and re install it of course - connect the user on others computers in different network's node and the slowness is every where. We also change the user's computer. - check the network connection - check the database and put agent on database for index ... without success.

Today, we didn't know what we can't do because our knowledge is not infinite on Nav and we search a durable solutions.
Do you know a way to detect the source's of the problem, an idea or a solution ?

Thanks for your time,