Weird flatspot at 4600rpm, can't 'break through' it in 4th Options

[nebreitling]

hi all

i've discovered a weird ignition/fuel system quirk in my '75 2.0, which has probably been going on since i've owned it (3 months) -- just haven't noticed it.

the engine has a flatspot at ~4600rpm -- it feels like the fuel cuts in and out to keep the engine below this speed. now, i can 'break through' it in 1st --> 3rd gear, it just pauses and stumbles a bit at 4600, then moves merrily along toward redline. but in 4th (and 5th, i presume), it feels like i don't have enough power/momentum to 'break through it'.

2 months ago, car had a full tune up, valves adjusted, etc. airpump removed, air injector jets plugged. aside from this quirk, it runs smoothly and strong.

does anyone have suggestions for things to check? (IMG:style_emoticons/default/confused24.gif)

nathan