1998 Honda Accord engine heating up

I have a 1998 honda accord. 138000 miles and runs great. For a while the temperature of the engine has been going up. This is what I have noticed in terms of when it goes up:
1) when I run the a/c for a while and I am parked or stopped at a light for a long time.
2) if I am going uphill for a long time, even without the a/c
3) or just driving out in the summer time. And after about 20 or so of driving, if I am stopped at a light, I noticed that the temp was high.

so basically there has been no clear time when ti happens. I drove 600miles 2 weeks ago and there was no problem. Once I got to the destination, there were a lot of hills and I had to drive very slow with all the traffic lights and after about 15 minutes of doing that, the temp was really really. It may or may not happen in the above scenarios. Different each time. What could be the problem? And what might be the cheapest way to fix it. I would appreciate any and all help. Thank you!