Erratic/High Idle

[twitchyboy]

Hey Everyone,

The Problem 08 Toyota Yaris Sedan 5 Speed has erratic idle between 500 and 1000 rpms with it idling as high as 1500 when cold.

The Story

I recently purchased a 2008 Toyota Yaris 5 speed Sedan at ~82000 miles. The car drove great for about the first 1000 miles and idled at 500rpm. Before buying I took it into the toyota dealership and had them do a full inspection and they said it was in great shape - just would need recommended maintenance soon (replace spark plugs, change transmission fluid). I declined this, taking it to my local mechanic for a better price, but he couldn't fit me in.

So...because I travel for work, I took it on a 14 hour/900+ mile trip going 80-85 most of the way, the idle became erratic, ranging between 500 and 750rpm. Additionally, when the idle would range back down to 500, it would idle pretty rough, feeling like it was going to stall out.

After the 900+ mile return trip, the maint reqd light came on at ~84000 miles and I got the oil changed. I realize this is separate from the check engine light - just thought I'd add it.

After taking a ~750 mile trip going 85 most of the way, I noticed that the idle had started to range between 500 and 1000rpms. When the engine saw it's first really cold weather of the year last night (30s fahrenheit/0 celsius), it woke up and idled at ~1500.

After the car and the weather warmed up out in new mexico today (60s Fahrenheit/17 celsius), the idle continued to range between 500-1000rpm.

Other possibly relevant information:
1) previous owner drove it on a lot of gravel roads - could the extra dirt from this be clogging up the intake manifold or something?
2) Took it to autozone to scan for codes and they hadn't turned up anything

Next Steps?

What should I do here guys? Is there anything I can check on my own before I take it into the toyota dealership?