Car chokes/bog will only let me run from 25-30 mph

[VinceSez]

I have a 91 240sx (ka24de) with a weird problem that I can't seem to solve. When my car get's really warm it begins to bog/choke. When this happens the car begins to slow down and bog/choke around 25-30mph. The speedometer would usually bounch between 25-30mph when this happens. I recently had the head resurfaced as a result of a blown headgasket. I've also had a new MAFS put in, as well as a sensor located just behind and below the intake manifold. Does anyone know what the problem could be?

I also noticed that this started to happen more often after I installed my Hotshot Headers. Could my engine be choking as a result of the restrictive factory exhaust I have?