Lisa,
A couple of things I might try:
1. A large hospital grade pump with a double kit. I'm wondering if this
might help elicit the MER more easilly than the Nurture III because of the
cycling. What about breast massage?
2. A syringe and feeding tube or an SNS. The baby might keep up a stronger
nutritive suck with a consistent flow. This in turn may elicit the MER. If
the peri-syringes are helping, then it might be worth the investment in an
SNS.
3. Have the mom keep a detailed diary of when an MER occurs. Maybe, just
maybe, the diary will give you a clue.
Good luck!