Are you running the latest firmware on the synology? It is important as TM changed a bit with Lion and you need to keep firmware up to date to match the changes.. but Apple are a closed company now and they do tend to only make stuff within their own family work together.. there are other solutions than TM that may well work better with non-apple NAS.
To yonny, if you are using TC are you using wireless? There is no doubt that wireless has a lot more issues and if you have issues try using ethernet instead and see if the problem keeps happening.
Pondini has several suggestions.. but there is no single fix.
http://pondini.org/TM/D7.html
http://pondini.org/TM/D3.html
Read through the troubleshooting area.. as a lot of things can happen.
If you did a Lion update over SL it seems to be less stable than a clean install.. If it bothers you, I would seriously look at clean install.