Hiya Tom,
What this situation called for (as with so many situations) was MORE POWERRRR!!!!
The forces only push the particles so far so I added some more at key points. The bunching was caused by particles banging into each other and getting stuck so I turned particle collision off while leaving object collision on. Also cranked up the particle bounce a bit to help them keep momentum. I've attached the modified project file. Hope that helps with your experimenting.
Dan
blowing.mp4
WindyTunnel powwwerrrr.prj