iPhone always reboots after 3 min of use, NO IMEI, NO BLUETOOTH [...]

I'd take a look at the battery... it may be that it is not compatible in some way with your phone, or it is at the end of it's life. If the phone doesn't reboot when plugged in to the wall charger then this makes it a bit more clearer that it is the battery. (if you are testing/charging using the pc then the power supply is fairly low compared to the ac outlet)

Restore with the 4.2.1 custom file. first start the tiny umbrella then put the custom file into itunesby holding shift button to restore. after restore check the imei if have then no problem. if imei= unknown then its very difficult to rebuild the phone have to change the logic board.

It sounds like a corrupted/bricked baseband. You're best bet would be to flash the iPad baseband, and hope that it works.

if your 3'rd 4th and 5th serial number are higher than XX128XXX, and you install the ipad baseband, your wifi, imei, and bluetooth will be gone.

there are NOT several reasons for this (imei bluetooth AND firmware missing): It means you or someone with your iphone upgraded to the ipad baseband (6.15.00) using redsn0w in a failed attempt to unlock the phone for other Sim Carriers, and your iphone was made after week 128 (after 28th week in 2011). this is mostly the new 8Gb 3GS. Your phone is now useless because the baseband chip is deprogrammed. You will not be able to EVER install ANY baseband on these new chips again. I have heard the chip can be replaced by a process known as reflow resoldering but I have yet to see this repair done. Does anybody know the retail value of phones in this shape?

"Currently the only solution for this issue:

http://www.beijingiphonerepair.com/store..."