20,000 Patties Under the Sea/The Battle of Bikini Bottom - S5-E17
Audio problem: In "20,000 Patties Under the Sea" When Plankton says "Ah who needs that rustbucket anyhow!" his mouth is way out of sync.
Money Talks / SpongeBob vs. The Patty Gadget / Slimy Dancing - S5-E10
Audio problem: In "Slimy Dancing" in the scene where Squidward and Spongebob perform their act, we first see Squidward making Spongebob spin around. There's a shot where it shows Squidward inside Spongebob. The curtain in the background in the shot remains still, even though you can hear spinning.
Answer: I believe this is the Season 6 episode "Spongicus".