Engine rpms oscillating rhytmically

Hello,

I have an 73 911S and this evening, I suddenly experienced a strange problem:

The car starts and idles fine (cold and warm), revs up to 1900 rpms normally. Going over this invisible mark, it starts revving/oscillating rhytmically! If I hold the accelerator steady as if i would like to achieve i.e. 2500 rpm, it begins to rev rhytmically: up to 2500, immediately falling down as if it would stall, then catching itself just at idle speed, reving up to 2500 rpm, falling down... this is oscillating up and down, all while I am holding the accelerator steady.

This means I can only drive with rpms up to 1900 rpm, otherwise it gets to the point of undriveability.

The same thing happens if I try to accelerate with the hand throttle - everything over 1900, and the engine begins to oscillate. One up/down-cycle last for about 1,5 seconds.

Any ideas?
Thanks
Hannes

I suspect fuel delivery or an MFI adjustment (and I certainly don't claim to be an MFI expert).

But, I had a similar condition to what you describe - it was due to wires in the CDI box that had worn insulation from rubbing against the case. It certainly wasn't the first thing I thought of. The car idled fine, but had fits with anything over 1500 rpm.

First, I'd check the fuel filter, points gap, and coil.