40d and strange focusing results

I have had my 40d for a few months now and get some hits and misses with my AF its mostly spot on though so those focus misses havent bothered me to much.

I tested out live view today and decided to do some focus testing and had some strange results.

I put the cam on a tripod and focused on plain text in a book, The 70-200 2.8is hit the focus everytime no need to fine tune the focus. I put my tammy 17-50 on and now my problems started, I had some bad results, lots of misses. The strange thing is that if i put focus on infinity and then auto focus it hits every single time. Next up was the nifty fifty, it had some hits and misses also, if it hits focus and i auto focus right after it will miss, It seems the problem is that when the lens have to hunt for focus it misses but if it only have to travel in one direction it hits focus.

Anybody have any problems like this or tested this ?

i will do some more tests with my other body, 400d that have been calibrated by canon, will test with some more lenses also, to try to figure out what the hell is wrong.... It's not like i can call canon and claim i have a faulty camera when it behaves strange with a tamron lens

ohh and i used the middle single point to focus with in one shoot mode.

I'm guessing this is indoors, under (typical indoor) low lighting? That doesn't really surprise me, as both the Tamron and the 50/1.8 always performed badly for me under these sort of tests. They should perform a little better in more normal shooting, or with much better light, but low-light focussing is not very reliable on either of those lenses IMHO Still, at least they're cheap!