okay that's the proper way to check using a vacuum guage- mouth works fine too (not sure about the 8" vacuum). The only way I've gotten mine to stall is by opening it very quickly w/ my fingers on the diaphram. Aside from that the laboring idle is whats expected. Humm, maybe mine needs more cleaning, need to check the Helm's on that to see if it's supposed to stall.

I suppose the MAP sensor may be getting "tired" not reading the slight pressure difference - but that guess is just as good as saying it's the ecu. Possibly cheaper/easier to swap than an ecu. Borrow a friends, if you're lucky most hond/acura parts are the same, at least my wifes 97 CL and my 97 Prelude are a lot alike.

Oh and not sure what year your Prelude is but 97 (and possibly others) has an emission warranty extension 14yrs 150k miles. If you haven't done this extenstion replaces the plugs, distro, cap rotor, wires, oil for free. Not quite sure what else they look for but egr is part of your emmisions system.

Keep us posted on what you find...