Time Machine gone crazy - Why?

[Micky Do]

I am no "power computer user"; my early 2009 Mac Mini still serves my needs just fine, and I imagine will do so for several more years. Internet, music, photos (since I got a camera last June) and for work as a teacher (mainly Pages & Numbers) is all I do on a regular basis.

Work files are the only ones that change on a daily basis, but the changes are small. I add, and edit a few photos every week or two, but keep only a few. The 120 GB HDD still has over 81 GB available.

While using just Safari (which I have excluded from back-ups) I noticed that Time Machine was backing up so I opened it. I was surprised to see it was busily doing a 13.21 GB back up. A few minutes later the back up was up to 13.82 GB (screenshots attached).

I checked iStats, and saw CPU use was way up at 33%.... 54%.... more. CPU temperature was 71ºC and the fan was spinning at over 2,300 RPM.

I shut down Time Machine, and the stats soon fell to the normal over 90% idle, temperature 56ºc and fan at about 1,500 rpm

A few months ago I added 4 GB of RAM (1 GB + 4 GB = 5 GB of RAM) and upgraded to Mountain Lion, (both done at the same time at an Apple agent; I don't have a credit card so don't have an App Store a/c). Since then I have installed the updates as they came along, so now it has OS X 10.8.2.

Mid 2012, before the upgrade, the external 320 GB HDD I use for Time Machine back ups still had over 270 GB available. Now it is down to 17.8 GB, though it seems to fluctuate. A week ago it had over 50 GB available. A couple of weeks ago it was about 32 GB available.

Is there a problem? If yes, what is is, and what can be done about it.

Is it with the 10.8.2 update...... which was pulled a few days after it was released?

Any comments, insight or advice would be appreciated.

[Micky Do]

I ran Disk Utility, and it reported that all is sweet.... so the mystery remains!

I have had Time Machine mostly switched off of late, just turning it on once a week or so to back-up. There was less than 17 GB available space on the hard drive according to Time Machine earlier today. It backed-up and lo.... it finished up with 47.8 GB available!

Once again, comments, insight or advice would be welcome.