Sometines it helps to work with a layer instead of directly on the data.You should try to creatre such a layer before you run the tools (with arcpy.MakeFeatureLayer_management), then you delete this layer (with arcpy.Delete_management, deleting the layer, not the feature class, of course)  

It can also help to avoid multipart features using the "SINGLE_PART" option and to reduce the number of vertices using "[simplify][1] (you did simplify at the stage of the raster to polygon conversion.

Finally, there is a work around to dissolve if you have the advanced licence, but it takes time. Use polygon to line with the option identify the left and right polygons, join the original fields that you need for the dissolve, select all the lines that have a different value left and right, the convert those lines to polygons: You have dissolved your dataset.  


  [1]: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//007000000011000000