Rosbif,
It's not just you. The TTL on my A100 works HORRIBLE. 100% unreliable which is why I never shoot in TTL mode. I shoot with my flash in manual mode all the time. I've had scenarios where I was in TTL with flash compensation up +2 and the picture was still underexposed (regardless of ISO) (person had on a Light Beige Sweater)compensation still didn't work using kit lense. TTL still failed even when I knew I had focused on the subject (so distance info could be relayed back to the camera) and coupled with my aperature.. and ISO ... it was still a no go. TTL on the A100 just gives me headaches so I stopped using it.
So what do I do now? I Shoot manual and I carefully watch my histogram. If there is something white in my shot then some of the data in my graph should be about 2/3rds of the way into highlight section (4th region) on the graph but NOT touching right edge. That will give you almost perfect exposure on something white without it being blown out. Once you get an exposure making a graph like that then you can just shoot away and it will remain consistent. For really dark tones make sure nothing goes off the left side of the graph (unless you really want deep dark featureless shadows).
You can adjust the flash intensity in larger increments from 1/1 down to 1/32 of the power. Start at the 1/8 power and move up or down as you need. To get a more granular change in flash intensity you can manipulate your Aperature in 1/3's... and for even smaller adjustment simply move a few inches closer or farther away from your subject until they are exposed well. Once you get your correct exposure then you know that for "that aperature", at "that power setting" on your flash you need to be roughly "that distance" away from your subject. It is reliable and repeatable but takes some practice to learn how a well exposed looks on the histogram. I've learned that a bright picture on my A100 LCD (with a good histogram) will be really close to being properly exposed in jpg or raw.
To see what stuff looks like when correctly exposed then use a light meter or grey card to set exposure then look at the screen and histogram to get a feel for what "right" looks like. You can get to know your camera this well but it takes time.
This method is hard to do on fast moving targets unless you've figured out the correct power setting/distance in advance. A little testing before hand can go a long way.
I always shoot this way because it gives me full control on what needs to be tweaked. If the flash intensity needs to be decreased by just a smidget then either I will stop down my aperature by 1/3 or a move a few inches away from that person. Try it.. it works.... A small move will result in a small change in exposure if you keep the flash power set the same. Closer it will get a little brighter.. further away a little darker. You can test this with a light meter to see what I'm talking about.
One thing to aim for is to get a good exposure of your ambient light first WITHOUT FLASH. Then stop down your aperature 1 to 1 1/2 FSTOP then adjust your flash and distance from subject at that setting to illuminate them well. Once you nail it.. it will never change and all your shots will be consistent IF you stay the same distance from the subject. Just play with it.. it may take you a few pops to get it dialed in but once it's dialed in.. you don't have to worry about the camera messing it up and from that point you can just shoot and adjust however much you need based on movement. After a while it will become second nature on what to adjust. Aim to nail it right there on the spot but shoot in raw to attempt to salvage images that are off track and yes some will be.
I used this method of using manual flash to capture my son's wrestling tournament.. balancing ambient with flash.. from this picture to the end.
http://ivanwatkins.smugmug.com/gallery/4134959#241280655
Also check out this write up on flash it may help make things click.
http://planetneil.com/tangents/flash-photography-techniques/1-natural-looking-flash/
Try it... I hope that one day I have a camera that does TTL correctly but until then I will be all manual.
Ivan
http://www.ivanwatkins.com 
