Rounding to strange results

Hi, I use a product database for 2 sites, one of which adds sales tax and rounds to the nearest .10 and on the other site there is no tax and also no rounding. I am unable to figure out why a price of 1000.00 is being displayed as 1.00 and any price over 1000 does the same thing. 100.00 works fine, as does all other prices, this is the same with and without rounding. Anything I should look for to solve this problem? Martin.