HL2 combine truck behave differently, WHY ?? Thread

Made this level a year ago or so, and the combine trucks did drive well then, one after the other one
they come together. As can be seen in this movie at point 1:00 minute;
Youtube.com

Now they suddenly lose there path and speed seems to be wrong.
as can be seen in this new gameplay video;
Youtube.com

But how can this be, because the other movie shows that it worked fine,
after many weeks of work. How can a level that isn't touched change over time .... ?????

And more important, what can i do to fix this?
Leon

Am still not really sure but i suspect that the APC trucks behave differently
when they reach a pathtrack were the speed is changed. have to look in to
this a bit more though.

Should someone know a solution of why this happens then i still am very interested
to hear that....

Leon