Important: Do not flash the modem with the phone plugged into USB or the charger. It could cause a no service at restart. Another note: If you flash a modem that has 850mhz enabled by default...you need to start the whole procedure over. If you do otherwise it may not work. So it is at your own risk.....
Step 1) You need to flash or be on (UGJL2, TLJL3, UGJK3, UGJK4) and enable 850mhz ONLY by going into the dialer and type in *#*#2263#*#* select WCDMA and select 850mhz.
Step 2) Boot into recovery and flash (BVJJPD or BVJJPG) then boot up and type in the dialer *#*#0011#*#* and verify its on 850mhz.
Step 3) Reboot into recovery and flash (XXJVK or ZSJPG or XXJQ1 or BUJS1 or XXJVE) then boot up and verify its on 850mhz by entering *#*#0011#*#* in the dialer. If you don't have 850mhz you did not do it correctly or your phone/ROM doesn't like the modem.
Step 4) Once you verfified your on 850mhz, go back into the dialer and enter *#*#2263#*#* and select automatic then select menu> back. Then select combi bands and then select to add wcdma850. Now select menu>back and verify the * is next to all bands. If so press the back to to exit and let the modem do its job.
At this point more than likely the phone will stay only on 850mhz until you reboot.... then all the other bands will work properly.
NOW: If you are using an i9000 modem such as (tubjv6 or tjjjv5) etc..... there is no need to do the modem dance. I do recommend that you don't manually force the modem.....let it do its job. Now before you flash from these default modems..... including if you do the dance above...... there is no need to force them to flash them. BUT, you do need to make sure that the phone is on 850mhz. So *#0011# and verify in 850mhz, if not, make it. Go into a spot where signal sucks, cover the phone with your hand... do whatever to get it into 850mhz. As soon as that happens.... reboot into recovery and flash. The reason why I recommend this.... the modem does a better job of switching to edge and gprs if needed. Overall I feel it just works better.