Wow, you've really been waiting for a reply!
It sounds like a worn tranny to me. I don't think a replacement would be a whole lot of money though, if you find a good junk one.
Slipping under load can be the clutch, but slipping out of gear and engagement problems can be in the tranny. The clutch being excessively worn will make engagement difficult, but once engaged, a clutch shouldn't make it "jump out of gear".
A common problem is the 3rd gear synchro which wears and cause a bit of "crash" when engaging 3rd. But that can affect any gear.