You misunderstood the meaning of "Prelim Code" in an ODE page. And also the need to correctly initialise the model variables either in Variables or in Initialization.
You MUST give correct initial values to the variables as a result of one of these: - the Value cell in the table of variables - code in one of the Initialization pages - code in one of the Fixed relations.
If not, the variables can have ANY random value. Typically, Java gives them an initial value of 0. But when you click _reset(), they 'probably' will keep the value they had in the last run. Hence, the different runs you are seeing.
Now to the Prelim Code. This is not code that is run once before the ODEs are solved. Nope. This is a placeholder for lengthy code that you need to do to evaluate the Rate values of an ODE. It is run, probably many times, before evaluating whatever you write in the Rate cells of the ODE.
Therefore, it is NOT a place to locate your initial values for the variable, as it seems to me you are doing.
I suggest that you clean your code. Move all the initial code (now in Prelim Code) to one Initialization page and keep the Prelim Code to its minimum. In your case, maybe empty, since the ODE is rather simple.
