iOS animation: background continuously moving (game)

I am currently coding an educational game in which I need the background to be continuously moving. I've tried to use blocks (iOS 4), for some reason the result isn't satisfying (even if I specify a linear animation, the image slows down at the end. Any suggestion? Thanks for your help!

    soil0 = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"soil900.jpg"]];
soil0.frame = CGRectMake(0, 0, [UIScreen mainScreen].applicationFrame.size.width, [UIScreen mainScreen].applicationFrame.size.height);

[UIView animateWithDuration:5.0
                      delay:0.0
                    options:UIViewAnimationOptionRepeat | UIViewAn开发者_如何学JAVAimationCurveLinear
                 animations:^{
                     soil0.transform = CGAffineTransformMakeTranslation(0, -450);  } 
                 completion:^(BOOL fin) { if (fin) 
                 {  
                     soil0.transform = CGAffineTransformMakeTranslation(0, +450);
                 }
                 }];

I think perhaps you are seeing easing. UIViewAnimationCurveLinear is for use with the setAnimationCurve class method. Try using UIViewAnimationOptionCurveLinear instead.