No need to replace the sand unless it's gummed up from something like a baquacil to chlorine conversion. Bubbles in the returns indicate a suction side air leak of some kind. Could be a tiny crack/slit in the hose to the vacuum. Just possibly when the thing moved into a certain position this slit opens up. Does everything continue to run OK without the cleaner? Best guess for now.
Also, deleted your test post.
Al