E21 320i gear shifting problems, grinding?

Hey, im new here and im looking for some answers. I have an orange/red bmw 320i e21 , its a 1982, beautiful car but one problem, 2nd and 3rd gear grind, the weird thing is tho yhe gears perform smoothly when engaged and if the car has a low idle it shifts just fine. The grinding occurs when i feel the urge for speed and i gain speed and i dont let my foot off the gas completely when i clutch so i can get a good launch in my next gear therefore the engine is reving a little higher than usual when i shift. But lets say i wanted to red-line it, it would grind terribly at about 8k rpm's but if i get to 8rpm's and let the idle drop before i switch it switches smoothly. I am so baffled. If you dont understand something let me know i know this sounds confusing but i am veru curious
Thank you,

Alex