This is an on going problem with some equipment better able to cope with heat than others!
1st off: design, try and put the controller in a position where it is in shade whilst the sun is hottest (mid to late afternoon?) There is a site that goes out mid afternoon as the controller is exposed to E-S-W sun.
Look at installing some circulation fans with a thermocouple (RS do these) they are reasonably cheap, and whilst will not cool the controller, will circulate the warm air and stop the components mounted higher up getting too hot. If mounted directly over the problem equipment, the fan should stop the effect of overheat.
Try and mount ancillary equipment lower down, obviously heat rises and equipment mounted at the top of the controller space will get much hotter than those mounted at the bottom of the rack.
As most power supplies are now outsourced by suppliers it should be relatively easy to get the supplier to install an uprated PSU (if this is the problem) most milspec equipment is rated to 80C.
hope this helps!!