I would try to rule out positioning first; rule that out because it's the simplest.
Also tightness of the strap. I think one guy said sometimes he got going really fast, enough that the wind flapping his shirt around caused the sensor to move out of position. Obviously you don't want it too tight, but it should be fairly snug.
For lube/wetting: the above recommended stuff is probably OK but you can also use inexpensive and readily available saline solution for contact lenses. Personally, I've always found my own sweat to be more than sufficient, even with a very hairy chest
Interference is a possibility if you are near others that have HRMs. Not sure if any ol' wireless device could cause interference; maybe. If you notice this problem consistently at a particular location on your ride, you might check that out (may not be much you can do, though). EMF disturbance (high voltage wires) could do it too.
Could be a defective unit.
Very unlikely, but not impossible: you may have arrhythmia (irregular heartbeat) or other abnormal heartbeat which can confuse some HRMs.
Polar has some info (not necessarily specific to Polar HRMs) that may help: http://support.polar.fi/PKBSupport.n...1?OpenDocument