According to this Press Release from the Mars Exploration Rover site, the Mars Rover Spirit is suffering the equivalent of Epilepsy.
Approximately every hour, something goes deeply wrong, and the system resets.
This could be due to any number of possible causes - hardware problems causing periodic discharges ( an RC circuit ), or more likely, the Software is periodically getting its knickers in a twist, throwing up its hands in horror, and saying "I Give Up". If the latter, the cause may occur well before the actual event, depending on the Software Architecture. One thing's for sure, remote debugging over interplanetary distances is Tricky.
Oh, by the way, when they first tried to communicate with the Rover, we were having the most spectacular thunderstorm I've seen in 20 years of living in Canberra. I counted 53 strokes over a period of 2 minutes at its height, mainly chain and sheet lightning, though we got a ground strike on a tree about 50 metres away.