Why is my car not accelerating after third gear?

My car will go fine until it gets into third gear, it seems to stop accelerating as smooth and won't go into fourth until I'm going about 50. When it finally goes into fourth gear if I try to accelerate more it will try to downshift back into third gear. I have no check engine light but the manifold has a crack (if that could effect it at all please let me know, should be getting it replaced soon -image above). My knowledge of cars ia fairly limited so any advise is helpful. Thanks!