Just had a JPI 900 installed in a 63 C; pretty cool EXCEPT...The manifold pressure is jumping around +- 1 inch, pretty quickly.  The gauge was rock solid before the install.  I read on mooneyspace about a snuffer valve to fix it and called JPI about it.  JPI said the sensor is super sensitive and they sent the snuffer free.  Mechanic just installed it and now MAP is stuck at 27.5" and wont move...   So now JPI says they're going to send another sensor, less sensitive, different manufacterer...