Ok, let me try and rephrase it then... ;p
Do you have
Fraps? If you don't, download it. You need this program to show your FPS counter ("frames per second counter", displays the amount of frames been shown each second as a numerical value, by default on top left corner in yellow text) while your playing. Get into SAMP, then type /fpslimit 100 (make sure frame limiter is on in your GTASA display options) and start playing for a while, taking note of the values you get on your FPS counter. It should average out at a certain number, so you just need to watch the FPS counter to figure out this number. Once you got the number, do /fpslimit [
the number minus a little] (so if you averaged out at 50, do maybe something between 45 and 50). The reason you minus a little is because your fps is not constant, it will get a bit higher or lower depending on what your doing ingame at the time, so you need to compensate for that. If you do this properly, you will get smoother gameplay without the weird slowing down lag effect.
I hope you understand now.

One thing to note is that your visual effects settings on GTASA may have a huge effect on your FPS, mainly if you got dynamic shadows.