I just tested by calling the clerk at my local convenience store, just so I could try a different number. Same issue, perfect for 10-15 seconds, then they can't hear/understand me. The clerk put her phone on speaker so I could hear what I sounded like. You could hear the first syllable, then nothing, then a few seconds later another garbled syllable.
My phone setup: Optimus V, aospCM7 (04/27), mmarz/franciscofranco's tweeks
GrooVeIP settings: Allow 3G/4G (chkd), Accept calls on answer (chkd), Dialer prefs-Always ask, Mic gain-0, Ringtone Override-Bell, Keep Screen On (chkd), Outgoing Audio Processing (chkd), Incoming Audio Processing (unchkd), Audio Routing (unchkd), Audio Mode Fallback (unchkd), Override STUN Server (unchkd)
Running Services: GrooVeIP, ScoreMobile, SwipePad, DSPManager, Google Services, Swype
Any other details I can provide, just let me know. Would a logcat help?