I would hate to see you give up. I'm sure that you've explored it ad nauseam, here and elsewhere, but here are a couple ideas that you might not have considered (without going too far afield from this thread topic):
- When I get the occasional algae expansion due to some disruption, and if the algae is susceptible to glut (red algae), I dose about 2 ml/gal of Excel to my tank for a complete knockdown. You may not be hitting it hard enough. I have posted, elsewhere, the preparation needed and process. Of course, killing it is only a short-term solution.
- There are environmental testing services that will identify the algae type. They can be expensive at around $100 but, if you're desperate, maybe it's worth it to you. Two that I have found are: