AFAIK, 3.5G and 3G doesn't co-exist on a BTS (Mac-hs architectural changes needs to be done for HSDPA/UPA). If you are seeing a 3G to 3.5G switch, I believe it should be a serving cell change or you are downloading something after staying inactive for a while? If you are facing difficulties in accessing 3.5G network, it should be because of network overload - though I agree it can't happen always but you've also mentioned that you are able to re-register on to 3.5G network after a while. Your operator should be able to fix it if it is a bug at their end once you let them know the issue because I don't see any reason why a particular phone should be denied access to 3.5G network...