Bug? MapServer as an html <img .. > source

[Ken Lord]

I use mapserver to generate thumbnail images in query result pages, using
mapserver as the source for an html <img ...> tag.

This has worked fine in MapServer 3.6 CGI without errors. I've used
variables passed in the URL to layers with filteritems to have the queried
features shown alone with background layers, used URL parameters to modify
the map file dynamicaly to embed and resize the scalebar, and added a cross
hair to center of the image using the map_webuser.. URL parameters.

I'm now trying to do the same with MapServer 4.2.0 CGI. using the same
polygon dataset as I was using in 3.6. It is not working consistantly.

It will work generating an image for some polygons, fail - not returning an
image when other polygons in the same layer are queried, and work for other
polygons in the same layer if you click in just the right place within
them.  For all these queries, all the other attributes queried are being
returned correctly in my template. Restarting the server had the effect of
shuffling the polygons that would and wouldnt work.

I have simplified the image source string down to only the problem layer,
with no filtering, and only the parameters necessary to create the image.

Simplified, the img tag looks like this:

<img src='../scripts/mapserv.exe?&mode=map&mapext=XX XX XX XX&map=[map]
&map_SIZE=125 75&layer=Forest_Fires_1965-2003'>

The mapext gets filled in by javascript.

If I change it so that it displays some other background layer, but not the
layer being queried,  the image returns properly everytime.

The layer within the mapfile looks like this:

LAYER
  NAME Forest_Fires_1965-2003
  TYPE POLYGON
  STATUS ON
  DATA "nt_FFIRES_a"
  HEADER "results/Forest_Fires_1965-2003_header.html"
  TEMPLATE "results/Forest_Fires_1965-2003_results.html"
  TOLERANCE 2
  TOLERANCEUNITS pixels
  PROJECTION
      "init=EPSG:26910"
  END
  CLASS
      NAME "Forest Fire Areas"
      STYLE
     COLOR 200 110 110
      END
      STYLE
     OUTLINECOLOR 100 100 100
      END
  END
END

I can't figure out why it works sometimes ... instead of always or never.
It is the identical dataset that works everytime for the same use with
mapserver 3.6.  The shapefile is about 9mb of very irregular polygons.

Is this a new bug? has it already been fixed in 4.4? Will I have to keep a
set of 3.6 files in place just to make the thumbnails?

Any ideas would be appreciated, the thumbnails add a lot of power to the
query result pages.

Thanks,
Ken Lord
Vancouver BC