char numstr;
unsigned long num = 500;
sprintf( numstr, "%lu", num );

You seem to have an error there. Try defining numstr as something like:

char numstr[10];

You need to define the length of the numstr string, otherwise the compiler assumes 1 byte. And that's only enough room for the NULL at the end of the string not to mention any characters that are output by sprintf.

Craig Steiner