I have had this problem 4 times with different cars for different reasons.
1/ My dads Mitsubishi Galant. It would run nicely until asked to accelerate hard. I found the disposable in line fuel filter was 95% full of water. It probably had never been changed in his ownership. New filter fixed this. A similar effect would occur if you had a rusty [internally] fuel tank with the fine rust particles choking the filter.
2/ The TR7 was intermittently flooding on the front SU after fitting a new fuel pump. I kept adding extra cardboard gaskets between the pump & the block to reduce the fuel pressure. By the time I had cured the flooding, the thing would go flat/hesitant at about 3500 RPM, & not rev past 4000. I decided the problem was the much higher fuel level in the front carb, due to miserable Triumph bean counters not setting the float chamber horizontal to the carb throat on the 7, as they had on the old Vanguard 4 pot, & fixed that. No further problem with flooding at 2 PSI fuel pressure.
3/ Fitted free flow air filters to the 7. Accelerating through the gears, the engine would go flat at 3500 in third. Had to go up to BAL needles to fix this.
4/ Fitted a Morgan air filter to my new 1962 Morgan +4 [1991cc TR3A engine]. Due to the shape of the bonnet, this was a box, with a single filter at the rear end only. at 4250 RPM the engine went flat, & would not accelerate, due to the back 2 pots taking all the mixture the single filter could pass. I believe this could happen with a dirty or wet filter on the TR7 filter system, particularly with a tuned engine.