I'm shooting normal 30i (scene file 1) and have not seen this issue before. My concern is that the camera may not be tracking the timecode properly or something else might be at work.
Thanks for your note.
Wait even longer to call action, call out camera...camera op starts recording, when the timecode starts running, he calls "Speed"...and then do your slating or whatever you do, and then call action...so there can even be 10-15 seconds of footage before you even start action.