my case is, if i slam the stem on my bike, i am a bit more powerful within 6~70km with no discomfort at all, but once over ~70km my shoulder start to feel tight and tired, and my hands start to numb. if i raise my bar by ~5mm, i just can't quite get the power out, and my lower back will feel tired very soon, especially on climbs, but no other discomfort at all, and i can happily(kind of) keep going for ~220km. my height is ~178-179cm, inseam ~82cm, bike is ridley helium size small, stem is zipp 120mm -17 degrees(have tried a 110mm -6 degrees stem but start having lower back pain very soon), and i move fwd and aft on the saddle quite a lot, on steady climbs i usually move a bit aft on the saddle, on steep climbs or on the flat with full gas i like to move fwd on the saddle. i did sit squarely on the saddle. any suggestions?