'98 Subaru Legacy Outback overheating

Hey everybody,

So I have a 1998 Subaru Legacy Outback. I know it has a pretty significant
exhaust leak and a check engine light P0440 code for emissions. I'm also
moving to Wyoming atm. So I bought the car about a week before I left for
Wyoming and used it to deliver pizzas for a few days; no problems. Then I left
for Wyoming with plans to stop in VA, IN, and CO on the way. I made it to
Richmond, VA with no issues, the car ran like a champ. On the way from VA to
Indianapolis there were no issues. However, while driving from Indianapolis to
Denver I noticed while stopping for gas that the temperature gauge was
redlining. Throughout the rest of the drive I kept an eye on the gauge (15.5
hours) and it never once overheated while on the highway. Every time I stopped
for gas or food the car would start to overheat pretty quickly but as long as I
was driving over like 40mph it was fine. I also just let the car idle for 50 minutes
and the temperature gauge stayed right in the middle where it was supposed to
be.

I've had head gasket problems with other cars before but I'm not sure thats
what this is. The oil isn't milky, and I haven't noticed any white exhaust (tho the
exhaust leak could be messing with that).

Like I said, the car runs normally while on the highway and while idling but if I
get off the highway and drive to a gas station or something it starts to overheat.
Any ideas?
(sorry if anything is unclear, first time posting to a forum. Feel free to ask
questions)