Chrisc:
I'm sorry you're having such a problem with your lens. I haven't heard of a problem quite like your before. I recall the first time I realized there was an adjustment on the zoom resistance; for a few brief, horrifying moments I thought there was something wrong with the lens.
Anyway, all my problems with the 100-400 have been self-induced (dropping it), and the Canon repair people have come through quite well for me, albeit $300US later.
I have the 100-400 now (actually two of them), the 70-200 f/2.8L IS, and the 300 f/4L IS prime. I can tell you that the 70-200 with 1.4X and 2X TC will give good results, especially the 1.4X TC, but not as good as the equivalent focal length on you 100-400. The 300 prime will be the best of the bunch at that focal length, but is clearly limited in versatility.
If 300mm is a useful focal length, it's a great lens and I encourage you to buy it. I would be reluctant to recommend the 70-200 w/TC if the bulk of your shooting is at greater than 200mm. Even with the 1.4X TC, the quality will be slightly inferior to the 100-400 zoom.