This sounds like the problem I have with P7.  If this is the same thing then those streaks are actually holes in the alpha channel.  Altering the number of threads used and lowering the bucket size can help -the holes are usually limited to one bucket- but I could never get render settings that would eliminate them altogether for every situation.

I contacted efrontier tech support about this when I first got P7 shortly after SR1.  They told me that it was a known issue that was fixed with SR1 for most people, but not me.  They offered me no solution or advice other than to use the P4 engine.  I began using the P4 render engine, which is faster using one CPU core than Firefly is with four, and in my opinion just as good, unless you need things like advanced AO or raytraced reflections, etc...  Unfortunately P4 doesn't handle the advanced shader tree, and it was fairly crash happy.

Recently I had a project where I had to use figures that used the advanced firefly shader nodes, which render as distorted rainbow colors in the P4 engine, also P4 would crash after rendering animations for a few hours (A massive waste of time).  So, back to Firefly, and the streaking problem which still hasn't been fixed as of SR2.

Again I contacted tech support about this.  They told me this is a known issue that can happen when the camera geometry intersects other geometry in the scene.  I looked over my scene to make sure the camera wasn't intersecting anything, it wasn't.  I noticed for me that the holes are patterned on geometry behind the camera.  I tell tech support this, they tell me that this is a "known issue" that is scheduled to be fixed in SR3, which is due out later this year (that's as specific as he got).  He told me he would try to find a work around and get back to me, he never did.

When I turned visibility off for objects behind the camera, the streaks went away, so that is the best solution that works in certain situations, but not all, obviously.  At least with this way I can get a streak free render without tweaking the render settings endlessly.

If you're familiar with the recent history of poser, you'll know better than to hold your breath waiting for SR3, or P7 Pro.

My best solution was to get Carrara 6, its not totally in-line with poser, but it is very good with poser content (as long as you don't need to use poser 5+ shader nodes) and Daz content is fully supported.  The best part is that the render engine is light years ahead of poser.  The render engine, on my computer, renders 10 to 20 times faster than P7, no exageration.  Image quality is also better on carrara default settings than posers mid range settings.  You know how poser ray traced shadows are harsh and distorted around the edges unless softened?  Not with Carrara.  You can also look at Vue as an option (which can render the advanced poser shaders).  I mentioned Carrara first because the product line is cheaper and the render engine speed really does blow me away.

So, to sum up your options  (And I want to point out I had to come up with all these myself,  except for no.1  Poser tech support gave no other help.):

1.  Use the P4 engine.
2.  Test render and tweak the firefly settings endlessly until you eventually get settings that produce a usable image.
3.  Turn visibility off for geometry that is behind the camera.
4.  Get Carrara or DS or Vue and render in that.

I eventually settled on 4.  even though I'm ticked that I have to spend extra to get something that should have come with poser in the first place (a working and stable render engine).  I have to say the money spent on Carrara is worth it in the time it saves over poser's slow-as-molasses render engine.