I need advice on how to fix engine hesitation @ 3500 rpm.

I just got a 86 rx7 gx, and the engine hesitates at 3500rpm to 5000 rpm. does any one know how to fix this? the motor runs fine other than that. I am new to the rx7's, but have always wanted one, and I would like to do it all myself. It is now my summer project.

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  chefsanctus
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  Scott3784