I had the same issue for decades. Vitamin C made me sick. It even gave me bloody diarrhea at times.
I found Solgar's Ester-C Plus to be the only Vit C I could tolerate. It is calcium based so it is gentler.
I also found that once I treated for parasites Vit C did not bother me anymore. I can now take 6-8 grams without a problem were before anything at or above 1000mg made me sick. I still get the "C BURNS" as I call it, if I drink too much juice with C in it.