Lag spikes tend to be caused by one of two things: 1.) Programs running in the background that periodically use the network or harddrive (p2p, some AV scans, widgets, gadgets and extra crap...) 2.) Something interfering with your wireless networking (if you use it). Most cordless home phones tend to cause interference as does keeping a mobile phone near your computer or the router.
Chances are these spikes are happening all the time but you only notice them indoors, as a lot of machines seem to slow down indoors. keep an eye on your lag-meter as you run around town... see if it's only indoors.