'91 M50 5-speed ~200K miles

Car operates great at 0-50% throttle.
Step on the gas beyond that point, performance is the same.
I cleaned the MAF and that helped some of the low speed sluggishness and made the engine more responsive.
Idle is very smooth, revs up to redline with no issues but feels down on power with throttle openings greater than 50%
Yesterday I reset the computer but it still performs the same, flat.
There are no warning lamps illuminated on the dash.

Does anybody have any suggestions on where I should start diagnosing the problem? I was thinking to test the resistance of the TPS potentiometer to make sure it is working properly but other than that I'm not so sure what could be causing the issue.

Has this situation happened to you?