Well you said you were having trouble with the ending. I'm assuming you mean the 2nd theme (the part that is mostly in 5/4 and descends from C to B to Bb, while playing harmonics too).
The key is to hold the low notes (C, B, Bb) while letting the harmonics ring out above. Firstly, I play nearly every note on the E string with my thumb while using my fingers to play the harmonics on the other strings. I feel this makes it easier to play the chords. I fret C on the 8th fret of the E string with my pinky finger while using my index and ring fingers to play the harmonics across the other strings. I then do the same thing except for sliding my pinky down a halfstep to B. And then I switch to my middle finger to fret the Bb while once again using my index and ring fingers to play the harmonics. IT'S REALLY ANNOYING and took me forever to get my fingers in shape enough to pull it off. The next part is similar. Go between fretting G# and G and playing harmonics with your other fingers. For this part I consistently fret the G# and G with my index finger, using my middle and pinky (and occassionally my ring) fingers to hit the harmonics. And ofcourse the outro is just a reprise of the beginning, except with a tapped artificial harmonic at the end. Bar the 9th fret across the A, D, and G strings while quickly tapping the 13th fret across the same strings.
Being exactly in time isn't super important, the whole piece pretty much has a free time feel to it. You might hate yourself for awhile, but as you get used to the fingerings it'll come together. I've seen it played with various different fingerings, so don't feel like you have to use mine. Use what's comfortable for you.
This was actually good for me too. I haven't played this in forever so I had to go back and almost re-learn certain parts. Nowadays I find making my own solo-bass pieces and music more enjoyable than playing someone elses. So as a recommendation, don't become one of the tool's that sits in Guitar Center and plays Portrait Of Tracy for 2 hours, trying to show off. Learn the song, and then apply it and what you learn from it, to making something original. I hope I've been able to help. Let me know if you have any other questions.
