Weird Iso-Contour behaviour

Hello,

has anyone seen some weird behaviour with iso-lines as shown in the figure:

http://www.geogr.uni-jena.de/~p6taug/weird_1.jpg

The iso-lines go off to the bottom right corner. First I thought it was my data, which have missing values (mapped to -9999). But it is not really the -9999 or the missing data (NaN), because zoom out still shows the lines ending up where a rainbow ends: nowhere.

See also
http://www.geogr.uni-jena.de/~p6taug/weird_2.jpg

which confirms that the isolines end/start somewhere *fixed* at minus infinity.

Furthermore, other simple test data containing missing don't show this.

Changing the parameters
http://www.geogr.uni-jena.de/~p6taug/weird_3.jpg

causes what, in my view, is abnormal behaviour: I set the base to 600 and suddenly only a few lines are drawn. (Lines are clipped in length, and not in height.)

Any views, ideas or at least some expression of pity ;-)

Cheers,

Ugo