I realize you've already checked the comments at Sparkfun since you posted a question there yesterday 
By the comments it looks like these particular modules are very sensitive to interference. I don't know if 315MHz is worse than 434MHz.
I've heard of wireless alarms and other equipment that more or less jam these open frequencies.
Maybe you could try your modules at another location to see if you have a problem in your area?
I've been using this type of modules for remotes and wireless sensors and I never had a problem: http://www.ebay.co.uk/itm/433MHz-RF-transmitter-and-receiver-link-kit-/261055772748
You can miss a packet of data once in a while but it usually works very well.
I open a garage door from within a car about 60 meters from the garage. Receiver and it's wire antenna is placed inside the garage.