Except when the frequencies are equal (normal interference), the pattern is NOT stable: the positions of the nodal lines (destructive interference) and

the antinodal lines (constructive interference) will shift with time. Nevertheless, the shifting is slow when the difference of frequencies is small.

Another applet which shows how the interference pattern due to two sources of slightly different frequencies changes with time

(http://www.ngsir.netfirms.com)