Power Factor sign changes from positive (+) to negative (-)

The sign of the power factor comes from the sign of the VAR measurement.  This indicates whether the load is leading or lagging.  A "perfect" power factor is "1". 

A slightly lagging power factor would be +1 (or +0.999), approaching "+0.000" as the power factor increases in lag.

A slightly leading power factor would be -1 (or -0.999), approaching "-0.000" as the power factor increases in lead. 

Essentially, the power factor oscillates around "+/-1", so this is perfectly normal.

This issue can be visualized by the image below:

Please refer to p. 47 of the manual for more information.  The method of calculating the power factor sign can be changed if desired.
http://elkor.net/pdfs/WattsOn-Mark_II_Manual_Complete.pdf