On the discussion about ROTJ:
I have no canon sources at hand to point to, but my understanding since 1983 when the movie and novelization came out was that somewhere between 6 months and a year had passed since the events on Bespin. In that time, Luke had not been back to Dagobah.
Which makes perfect sense, because when he does go back in the movie, he asks about Darth Vader being his father. If he had gone back earlier, he probably would have been shouting that question across the swamp as he jumped down from his X-wing. Of course, since he was able to 'search his feelings' in the Force, he already sort of knew the truth- he just wanted to hear it straight up from Yoda.
As to his training/ abilities. For the 3 years between SW and ESB, Luke was forced to muddle along with no true guidance other than what he got from Obi-Wan for a few hours aboard the Falcon. As such, he doesn't know much / can't do a lot. Conversely, after spending several weeks with Yoda on Dagobah he has been introduced to an entire training regimen and been opened to the Force. It makes perfect sense that for the six months to a year between ESB and ROTJ, Luke is able to grow and advance on his own by using the methods Yoda taught him. Since, according to Lucas at the time, Luke was the strongest Force-user who ever was or would be, it makes sense that he makes great strides. That's why Yoda decides he needs no further training- convenient, since he's dying anyway and has to pass the torch. As to him self-identifying as a Jedi, that's pretty natural too, if a conceit. Could be he's also trying to convince a powerful Hutt crime lord to take him seriously- and it doesn't work.
Anyway.
Oh, and there is a fanedit of ROTJ floating around out there that places all the Dagobah stuff BEFORE the Tatooine stuff, making it appear that Luke HAS been training in the interim, and isn't really exaggerating when he tells Jabba that he's a Jedi Knight and not to underestimate his power. It's an interesting take on the story, albeit totally not-canon.