The wing spar thing I mentioned before could be a factor. If it's big enough it basicaly acts like a spoiler.

Other than that, the CG might be a factor if you are massivly nose heavy, but you'd really have to be very very nose heavy, but I don't know.

If the streamer is too large, you might get something close to what you are getting, but you'd need a pretty big streamer to cause it.

You said the plane flys fine with out the streamer? Where and how are you attaching the streamer to the plane?