flat spot in powerband ?

[barthmonster]

My '91 Sedan with 4AFE (auto) runs excellent for what it is, but has a flat-spot with low power output in the midrange. It is smooth and torquey off the line and at high rpm but in the middle sort of 'dead'...

Of course this is an old car that had poor maintenance before I got it and I put in a new shortblock with the old head and cams. Possibly the fuel-filter, Oxygen-sensor, or dirty injectors ??

I know the above would be easy to just 'do' but don't want to start throwing money at this as it's my work car unless I'm sure it will fix it as I'm running good now and not bad on gas either...

Thanks !!