Is this normal?

[Runaboveit]

I was in the car the other day for the first time in HOT weather, it peaked at 110. drove across town and saw the temp gauge was almost halfway betwen the middle and hot but staying very steady.

Turned the car off, then 10 minutes later had to run out for some ice.. started it and it was at 3/4. Didn't want to drive it, so I took a friends car. Is it normal for it to run hotter in.. well.. hotter weather? I understand cars get hotter once you turn them off. On the way there it was mainly 55mph plus so I thought it should have stayed in the middle, but it wasnt and like I said steady at that mark.

Should I do a coolant flush? I opened up the overflow cap and noticed gross yellow brown gunk. I did top off the system with dexcool when I changed the thermostat back in December. Heater core is also bypassed. Lately I've heard a light hissing sound after turning it off. I'm thinking it may be the hose clamps on the heater core bypass. I dont have those quick connect fittings on them, just the hose clamped onto the pipes. Bought two quick connects, but havent installed them yet, will do that when I flush.

And if my research is right... if you have air getting into your system with dexcool it can cause it to gunk up like that.

My first thought was I was loosing oil into the coolant, but the oil level is right where it should be after I let it cool down and checked it.

The engine bay itself seemed really hot too. But this is the first time in weather above 85 or so, not sure what to expect..

Thoughts, Ideas? I know I'm going to need to do a coolant flush. Overflow looked pretty grody. Anything else?