My 2011 Honda CRV (180k miles) is having overheating issues. It's used for a mail route so I know that it's gonna be pushed to the limit. On a normal day my temperature gauge won't change and will stay 2 notches below the middle, but recently it's been moving further up than it usually does. When it would change it would settle at the middle between cold and hot and that's as far as it would go, and this is usually during a 90+ degree day.

Fans are working great, there's fluid in the reservoir, and the radiator cap isn't rusted out.  About an hour into my route, it started getting to the middle, then eventually beyond even that, almost all the way to Hot, and it wasn't even 80 degrees outside yet.

I take the CRV to the mechanic and tell them the issue, but they keep saying they cannot "simulate" the conditions, and therefore cannot do anything. I'm gonna try to talking with them again tomorrow with the pictures I took of the gauge and we'll see what happens I guess.

Any advice as to what else could be the issue?p