Similar thing happened to me. I had my Spider tranny rebuilt by a well known shop near LA about a year ago, and was experiencing this too.
The shop said my vinyl shift cover was too tight and was pulling it out of gear. This was not the reason, in my case, but I mention it because you should look at that.
Anyway, I just cut away my old vinyl shift cover, because it was really old, and it still popped out of 5th, and sometimes 3rd. Part of the clew to fix it for me was that 3rd and 5th are both forward, in the same direction, so I started looking at that. And with the benefit of the vinyl shift cover gone, I could see that the top-most portion of the "rubber" shift boot, which is really thick, was too high up the shifter shaft. It was hitting the console.
It was simply a matter of sliding the rubber down a little lower so that it went beneath the console when placed in 3rd and 5th.
Try a look at that.