White stripes in raster

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White stripes in raster

I try to render a raster from natural earth + modis snow cover (combined
with image magick) as a raster datasource, and I obtain white stripes on
certain zoom levels.
This happen with scale=fast or bilinear, but not with bilinear8, both
with originals from natural earth processed with gdalwarp, or processed
further with image magick.

The effect can be seen (temporarily) here:
http://tiles.pistes-nordiques.org/ with mapnik 0.71

The following setup is not the same, but also gives a subpixel line
almost every 6x6° on mapnik 2.1:

gdalwarp -of GTiff -overwrite -t_srs "+proj=merc +a=6378137 +b=6378137
+lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null
+no_defs +over" NE1_50M_SR_W.tif ne_m.tif

<Map srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0
+x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs +over"
background-color="white" >

<Style name="world-snow">
<RasterSymbolizer scaling="fast"/>
<Layer name="world-snow" srs="+proj=merc +a=6378137 +b=6378137
+lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null
+no_defs +over">
<Parameter name="type">raster</Parameter>
<Parameter name="file">rasters/ne_m.tif</Parameter>

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