Another issue with the ZR1 is that it has a "cinch" adjustment on the seatbelt which allows you to lock the belts so no slack is available during normal driving (presumably for hi speed driving).
The button for the control is a small rectangular red button near the bottom fastening point for the belt. Check that it works properly. In normal operation, it has no effect (assuming you don't touch it). When you use it, it locks the belt, so no slack can appear. repeatedly pressing it "ratchets" the slack out of the belt.
A couple times, I have had mine engage for whatever reason, and had the same sensation you describe, including having to release the belt to correct it. Make sure yours has the ability to stay out (as unpressed) when in normal operation, and I think you will be ok. You can verify this by pulling the button all the way out and temporarily fastening it for a whie. If the symptom is gone, that's it.
Regards,
Art in Baton Rouge.