Numerical weirdness with ode15s....

Hi, I am having a very strange (at least to me) problem. I have a code which is iterating through time spans, and in each time span, it will solve a set of time-delay ODEs with dde23 and then a set of normal ODES with ode15s.

Now if I start a clean MATLAB window and run this code, it goes for about 15 minutes with no warnings about solver precision. Then I call clear all to delete all output and rerun the code in the same window to verify reproducibility. I now get "Warning: Failure at t=5E+00 (seconds). Unable to meet integration tolerances without reducing the step size below the smallest value allowed at time t." for ode15s. If I just let the warnings continue, the code produces output for about 75 minutes.

Does anybody have an explanation for this numerical weirdness? And then how I can avoid it and what output is trustworthy? Thanks very much, Sylvia