Hi Martin,
I have the German version of the 650 (RM-1152) and had permanent trouble with WiFi since I bought the phone in February. I heard from Microsoft London that they are finally working on the problem.
Some weeks ago, I installed the preview version(s) of Windows 10 Mobile. Right now, I use the Release Candidate 14393.5 which is due for everyone coming Tuesday (at least as far as I heard). With this version, WiFi has been stable for a little bit more than 4 days now. This is promising as my longest time so far without a WiFi problem was close to 5 days. Keep your fingers crossed that they finally fixed the problem!
Good luck to you and have a great weekend.
Heiko