The start location is calculated based on the current SMPTE time received (from tape, for example) and the software's frame rate. If the software frame rate doesn't match the source frame rate, then the start location is calculated incorrectly and the session starts where it thinks it should. Things to look for:- Incorrect Frame Rate in software. It must match the source Frame Rate when recording and playing back. Some devices, like the DA88, set to generate 30fps, actually generate 29.97 when resolved to Blackburst. Also, a video tape labelled "30 NTSC" is probably 29.97fps.- The ATR/VTR may not be up to speed when Pro Tools gets a SMPTE trigger. Try starting the tape, then put Pro Tools Online.- Pro Tools and other device are not resolved to the same clock.- Possible problem with the Audio Interface sample rate clock. This can cause the sample count, from which the current MTC time is calculated, to be off. To test this, hook up the digital output of a DAT machine to the digital input of the Audio Interface and switch it to digital sync. If the sync improves, the Audio Interface might need to be serviced.