I am curious as to the engine not running smoothly after a while. I drive mine in 80-90F ambient temperature through the steep mountain roads here at high elevation so producing quite a load on the engine for up to 2 hours (then I need to stop) and do not experience not running smoothly, are you sure it is the heat?

Bruce
1964 4/4 Series V Comp (Megan)
1994 +8 (Maurice) tis no more
2013 M3W (Olga)