You've been caught by T-Mobile's stupid policies!
Your ROM and Radio versions are really out of date. They are the ones current when the device was released in July last year, and they were poor then. The radio is weak, and the GPS is very poor as well.
The idiotic thing is that T-Mo do have a later 1.93 ROM somewhere because warranty returns are sent back with it installed, but there is no way for ordinary users to access it.
As Dave says, there are many newer radios out there, but you MUST install HardSPL first, otherwise you can't upgrade any part of the radio or ROM.
Once you have HardSPL you can install anything you like, but be warned that technically this does invalidate the warranty. Take care which version of HardSPL you install first. I belive you can't put teh current version (1.93) on devices with the 1.37 ROM as your has. You may need to look for the 'Developers Edition HArdSPL V1.40' and install that first, then update the radio.
[Edit: I can recommend
This HardSPL from Jockyw2001. Use the 1.24 version- I used that right up to the later WM6.1 builds and it worked fine!]