What type of cam do you have?
If you are using a hot cam it should come on at around 4-4.5K so I would guess that you are having a transition problem! In my stock engine I was using 57.5 idle (actually they are transition jets as they do not greatly affect idle flow but greatly affect the transition to the main jets) so I think your 50's are small, especially for a hot motor. My 2.3L 911 engine in my 914 runs with 62 idle jets! It is ported and polished, 9.8-1 CR etc... So I can't help but think that your idle jets are causing your engine to go lean as it gets on the cam and then your 135 mains finally kick in bringing back full power. I would try 60's and see if the flat spot goes away.
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