I have the opposite issue, the garmin HR reads fine for the first 45 min ish then as I sweat more it goes mental, spiking above 200 regularly even at easy pace, but settling as I dry it and reseat it. I've changed sensor, batteries and the strap (onto a modified, cut, polar strap now) but it only puts of the problem until a bit later into the run.
Cheap ECG gel off ebay did help it me more reliable in the short term, but in general I've not been impressed with garmin HR monitors.
Edited: 02/09/2012 at 12:52