300tdi looses power if given too much throttel

So maybe someone can help. i understand that these aren't the most powerful engine however when I hit the highways I need to watch my Pedal. if while going up a hill i give more than 70% throttle not only does nothing happen between 70 - 100 but when the road levels out i actually struggle to build up speed again almost like the engine is being starved where is i control the throttle (by watching the boost gauge and giving it a little more after hitting 1 bar boost) the speed will drop the same but it will pick up easier when over the hill provided that i didn't go over 70%ish throttle if i did i need to wait a bit and slowly build up speed again. this is at speeds of 100 to 110km/h when i do "play" with the throttle carefully the car very quickly builds up to highway speeds even 125km/h and hold on medium uphills. just had the timing fixed and the valves set but still have this throttling on open roads issue. my goal is to cruise at 105 -110km/hand hold a bit better on the uphills and headwinds. also when this happens the difference between 4th gear and 5th gear is noise.