Ehh... Sorry, but You still don't get a point... :/ Ok, then, I'll try to show you something:
1. Build a model in Simulink, just as you wanted me to build, and in addition connect a Scope to the output of Constant. Put an 'a' inside a Constant as parameter, and set inf for a time simulation.
2. Create a m-file with a simple code just like this one:
tic for i= 1:1000 a=int2str(i) set_param('model1/Constant','Value',a); end toc
3. Set an a for a initial value a=0; 4. Start A Simulink model. 5. Start a m-file script 6. Wait for a script to reach it's end (takes approximately 1.4 sec. on dual core 2 ;) ) 7. Stop a Simulink model 8. Open a Scope...
And then you will finally know that the problem is... You see know? If still don't I'll explain one more time: For the beginning the model1 was running with a=0 and displaying that value... Then when you've started a m-file script it pauses... waits until it ends and continue to run with the LAST value of parameter a, which is in that case 1000. I can record a movie if it's still not enough clear what the problem is. ;)