Just had to reply to this, wife's 04' GT wagon has had these symptoms for a couple of weeks, been to the mechanic twice who checked for vacuum leaks, and added some injector cleaner to try the basics first. Next step was to pull out the injectors and bench test them as it was showing no codes with the scanner, so figured it was something mechanical and not an o2 sensor etc.
Pulled out the solenoid's last night and sure enough the drivers side filter in the banjo bolt had collapsed and a piece had lodged itself in the solenoid preventing it from moving. Couple of flicks with a small screwdriver and a small tap on the concrete and it came out, which allowed the solenoid to move freely again. Put it back together and all is well!
Big thanks to all especially bucky and cruisn for the pics which gave me enough info to tackle it in the driveway.
Needless to say the wife is stoked but still wont hand over the cash she was going to spend on the injectors!!