I've tried a bunch of different things to improve the wireless connection (new batteries, pull all batteries, do resets on all devices, move base unit) and nothing has worked for long. Am very close to sending it back (Oregon Scientific said the unit may need to be replaced).
I finally tried changing the channel on the spa thermometer from 3 to 2 and for some reason the temp has been updating properly for 6 hours now.
Voodoo.