He's certainly been one of it not the most exciting bowlers in that period, but I dont think he deserves to be rated above Pollock, Waqar, Walsh or even Gillespie. His contributions at times have been top notch but as you say he's very up and down plus he just cant stay on the park long enough to string a few consecutive series together and when he's bad he can be very bad. Its still not too late, but atm he's a bit of an enigma as a test bowler and I wouldnt put him near fifth.Shoaib Akhtar, I believe is a fifth place. Although his Test average is 25.30, his strike rate of 44.71 is second to very few bowlers who have played more than 20 Tests. In ODIs, the effect is similar with a strike rate of 30.17, going for under 5 runs per over, he is hardly expensive either. A very up and down bowler, many people in the world would pay to see the ups and the poor spells still come at an excess of 145kmph.
However in ODI's I fancy he'd be top 5 performers over that 15 year period.