[URGENT] Timing error (fMRI paradigm ends earlier than expected)

I am using Eprime 1.1 with IFIS to generate a fMRI paradigm for a visual task. Basically the task involves 2 alternating blocks, 1) 48 static dot pictures and 2) 4 cycles of 40 dot pictures (with slight differences in their dot positions). These two blocks are repeated 10 times each. Each static dot picture is presented at 500ms, and each picture in the second block is presented at 50ms. In total, the entire run should last 340 seconds including two fixations (8s) in the beginning and at the very end, and 2 dummy scans (2s/ TR).

I have found out that the run always terminates 8 seconds earlier than predicted time (332 seconds), and the final fixation was actually presented. Further, the dot pictures always freeze towards the end of their cycle (e.g. only 3 cycles of dot pictures were presented in block 2, and block 1 starts at its intended time). Freezing also happens occasionally to the static dot pictures.

I tried pre-release and preloading pictures but it didn't seem to help much. I have also set duration of dot pictures in block 2 to be 40ms (following that 10ms rule) but the fourth cycle still isn't shown. The pictures are in bmp format, greyscale, resolution 768 x 768. The screen resolution is set to be 1024 x 768, 60Hz. 

Is there a way or any in-line codes that I can reference to so that the pictures can be loaded faster? Has this problem occurred to any other users as well? The preloading example (eprime 1.1) uploaded here contains an unknown function so if anyone has any idea on how to preload 'better' or deal with visual paradigms that display pictures rapidly, please help. I still don't quite understand where the 8 seconds have gone, and why the pictures cannot be shown smoothly.

Feel free to leave a comment for more info.

Thanks a lot!