if you've been running for some time on the highway and then you park, the temps will tend to go up for a bit because the coolant is no longer circulating and the fan is not drawing off the heat anymore. it sounds like the radiator cap is releasing at that point so you may want to replace it first before proceeding any further. BTW what temps are you seeing on your gauge? in this warm weather, mine stays at 90 - 95C range. in stop and go traffic or slowing down after sustained highway speeds, your temps will go up to around 105C which then triggers the aux fans and that should cool it down quickly enough...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"