Trouble with presentation times

I'm having trouble with both consistency and duration of presentation times for my images. I have read through the sticky on being able to see subliminal images and even after following the steps listed there I am still having problems. The first problem I am having is that I cannot get my presentation times fast enough. I am trying to display images for close to 15 milliseconds. Using the refresh rate check I am getting 11.76 milliseconds at 85 hertz and 13.8 milliseconds at 72 hertz. I have set the resolution in both the default style and in the style folder with my my input folder to 1024 x 768. This is the same resolution as my monitor is set to, and yet I consistently get display times of 23 milliseconds when using 85 hertz and 26 milliseconds at 72 hertz. What I find interesting is that as I adjust the refresh rate of my monitor it appears the display time is usually close to double what it should be.

I have also noticed that even for my stimulus that are set to display for 500 milliseconds, their actual display time is often closer to 520. On top of that I am getting pretty dramatic fluctuation in how long my fastest images are displayed for ranging from 23 to 40 milliseconds. Around once a session one of these images will remain onscreen for up to 250 milliseconds. Have any of you experienced problems similar to these? I am using an older CRT monitor and have tried using a new LCD monitor as well and similar problems arise.

After reading through more posts I've changed my files from jpg's to bmp's and this has helped get the presentation time down on some of the images, but they still range from 12ms to 24ms, which is interesting because its still double.

Edit: I should also include that by chaning from jpg's to bmp's and then reducing the colour depth using Irfan I have gotten rid of the issue of the 500ms images displaying for too long, it is now just an issue of some images displaying for the right time (12ms) and some displaying for exactly double that time.