A 914 powered Highlander crashed once (I was there) with a very similar problem of "surging" during high throttle application and warm temps. The best we could tell after the crash (which was very severe both to the pilot and the aircraft) was that because the 914 gets all of its fuel from two electric pumps that continuously run fuel from the header tank to the engine , with all unused fuel going back to the header tank, that the fuel was getting hotter and hotter the more it was run on the ground, which caused fuel vaporization resulting in surging and eventually significant power loss right after takeoff. Lesson learned that day, run all unused fuel back to the wing tanks where there is a larger quantity of cooler fuel to mix with the hot fuel coming back from that engine compartment. Plus the air going over the wing also will help to cool the fuel, wheras the small header tank just lets the fuel get hotter the longer you run the engine.
I don't know for sure if that is why yours is surging, but the scenario sure sounds a lot like that Highlander. I'm guessing if it wasn't summer and if you weren't running it hard on the ground that maybe you wouldn't even know this was an issue. But now that you know that could be a possibility you might want to check out how yours is plumbed. The Highlanders were modified to return fuel to the wing tanks and many 914 Just Aircraft have since been operating successfully.
Knowledge is power...