Try this. Had a few issues with mine as well and after trawling over the internet, found this fix, which seems to work, at least temporarily.
Type *#52828378# in to the phone, then you will get a menu with the following options
1 Trace setting
2 OTA type setting
3 Serial test
4 TFS reset
Go to 3 Serial test and then you'l get another list.
1 PPP UP
2 Serial download
3 Run TCK
select '2 Serial download'
phone will reboot after that, and then connects to PC studio no issues.
Least that's what worked for me.