Had the same problem. Fixed it by re-rooting and flashing separately the DI07 radio update, followed by the DI18 versions of the radio, o/s and kernel in that order. (Do NOT flash a DI18 kernel first, it can cause problems updating the o/s). For some reason the combined file would fail to fully install and my phone seemed to want an incremental update from to DI07 radio first. You can find the stock files in .zip format for clockwork on XDA-Developers. DI18 is the latest official release at this time and helps maintain the 3G connection.....I was getting several connection errors during the day and even over night, hasn't done it since the DI18 update.

Oddly, even with the overlocked/undervolt kernel, PRL and PRI updates function fine. Some phones just don't seem to want to update off of DG27.