my.alpha = 62.5 + (37.5 * sinv(total_ticks*4));

Okay this is the weirdest thing... The lowest alpha should ever be is 25. It works fine on models but when I apply this formula to particals or sprite entities they seam to disapear as soon as they hit like 30ish... whats going on and how can i fix it?

I'm experiencing the same problem.
For debug pruposes i've setted the alpha directly and the sprites just disappear when i set the alpha below 50.

alpha 50 = visible, half opacity
alpha 49 = completely gone, invisible

I've tried every file format...
In entities works perfectly.

Perhaps this has something to do with the issue?

Well, i've tried PCX and BMP that doesn't have alpha channel. The alpha property should work on them, but, i've tried using a TGA instead and do the trick just fine.
Thanks for the jumpstart