What he said!!!
Start from the cheapest/easiest and work though it that way.
If you change a lot of things at one time you may fix it or make it worse not knowing what the result of each change is.
Getting a $20 temp gauge so you have more data to work with is important.
A lot of what we have to do is trial and error because we lack any real hard data.
Make a list of possible fixes/issues and then go down that list one after another, as has been suggested and you will find out what is happening.
Also check back and let us know what you find...
Good luck!