Engine rhythmically shutting off
 

I have a '97 Boxster, with the base engine & 5 speed transmission. I have done several things to it in the past year attempting to fix missing/rough running issues; replaced the plugs, all the coil packs, MAF sensor, and the 2 front O2 sensors. Ran great for a few days this last time (I did find a problem - one of the new coil packs went bad; reinstalled one of the old ones that I was pretty certain was still good and reset the ECU by disconnecting the battery).

Now it has started doing something completely different; it has never done this before.... you can hold the RPM's at a steady 2k for instance, and the engine will rhythmically cut out for just a moment, just as if you had turned the engine off and then back on.... on, off, on, off, on, off and so on. The dash stays the same however, other than the RPM gauge following the revs. It started doing this for a few minutes (embarrassing when driving thru town), then stop, and then do it again the next day and stop, but now the issue seems here to stay. So far it hasn't thrown a CEL.

And now of course, when I started it the other night it ran perfect. It will be fun to track down. Maybe its a good excuse to buy a Durametric analyzer?...lol

I sure would love to be able to drive it and enjoy being topless before the snow flies... any ideas?