Overheating

I have the wagon model and it drove fine for weeks when I first got it. It has 304,000 miles and it drives like a champ, but ever since it started getting cold in the mornings because fall is near my car has started to rise in temperature for a couple of miles and then go back down to medium range then it would rise again and drop again. So automatically I though thermostat replacement which I did and it drove okay for a day and now after about five minutes of driving it runs hot and does not drop back down. I do not know why it does not do that anymore it just stay right below the H on my dash. It does not run completely hot like there is no smoke coming from hinder the hood coolant is not spitting out from under the cap. And I have no leaks!
What else can I do? I have ran head gasket test cranked it while cool and full of coolant without the cap on and it did not spew out. My oil nor coolant look milky. So
I think it may be a clogged hose or seal somewhere in the coolant systems route
or bad water pump. Could some one give me some tips on what to do next? This is my only car and I cannot keep bumming rides to work. Please help.
Thank you,
Tim