MadTeam's CyanogenMod 7 restarting itself randomly
So, to avoid a hanging question:

Using the new ROM for almost two months. Tried the original stock baseband I5500BVJJP7 but it felt overall worse, so I went back to I5500LUYJPE. Now any issue is rare... it's weeks since I last noticed a random restart, my phone reports over 160h uptime (not 100% reliable, I've seen it not restart after a crash), and wifi is most of the time fast to get in IP.

In the end, as things improved with a new ROM, I suppose the issues are due to small quirks of the ROM itself. Should improve over time.