Its possible TPS has a Dead Spot, but you should notice that at any speed when gas pedal is in that one position
TPS is a 5volt sensor on the throttle plate shaft
With throttle closed computer gets just under 1volt from the TPS
With throttle wide open(WOT) the computer gets 4.5volts
So it works the same as a volume control or light dimmer, a variable resistor
If there is a "dead spot", voltage drops to 0 or under 1 volt in that one position, so computer "thinks" you took your foot off the gas pedal, and if you didn't then an unexpected stumble
But at 30mph or 60mph, speed wouldn't matter only TPS/gas pedal position would matter
RPM stumble, spark system
Again this should occur based on RPM of the engine not speed, so if its distributor or TFI issue you would expect the stumble in 1st, 2nd, or 3rd gear when at that RPM
At 60+MPH fuel flow would be at its highest, unlike 30MPH, or say 2,500rpm in 2nd
So fuel flow issue would fit better
Change fuel filter
Check fuel pressure at idle, 30psi is expected, then REV engine to 2,500(approx.) and hold it there, watch to see if pressure is slowly dropping, should stay stable at 30psi, if its slowly dropping there is a flow issue, which would be much more noticeable with engine under a load(driving)