QT: How to delete temp files?

This is doing my nut in!

I'm trying to convert a HD .mov file so I can watch it on my ATV. But I keep getting this error: qtKitErrorCatchAll (-2125)

Apparently every time this error arises the temp QT file is not erased and it is eating massive portions of my HD.

So I have two questions. What is the best way to convert .mkv HD files to AppleTV friendly format (without using Quicktime) and two... how do I delete Quicktime Temp files. If I need to use terminal can you supply the code and step by step instructions because I dont have a clue when it comes to Terminal.

Any help is appreciated.

Thats the download cache. I mean the temporary items cache i.e. files that have failed to transcode then disappear from desktop but don't actually get erased.