My phone gets bad signal- is it the network (virgin mobile) or the phone (sony ericcson w910i)?

Nokia have better reception than sony ericsson, but nokia also have the higher radiation level.

Motorola is the grand father of radiation.

This could be several things.

Software Fault

Sony Ericsson software can cause some problems as it is prone to problems, but a software upgrade may fix the problem.

Hardware Fault

You may have a faulty antenna in the phone.

3G/2G Network

Try changing the network to 2G instead of using 3G, see if this makes the problem go away

Sim Card Fault

You could have a faulty sim card. Try sim in another phone to see if the problem persists, if it does then get a new sim card from your network.

You answered your own question. "I've had about 4 mobile phones on Virgin Mobile in the past and haven't had this problem".

Sony Ericssons are junk. Change your phone. Get something more solid.

I've got the w910i and love it :) Its definately not big and bulky, its small, compact, easily slides into a pocket or bag and you cant "accidently" ring anybody as the buttons are covered by the fone having to slide open.

I would have said the network. But as you have been on that network for so long and never had the problem , it is probably your phone.

Go on review website - other peoples comments will let you know. Type in name of phone then add review on google.