If you are getting under-cutting and etching where there should be resist, I would not increase the temperature of the etch tank as my first step.
Are you using a positive or negative resist? If you are using a positive resist and get good resist removal, then increasing the UV exposure would probably make the problems worse.
Here are some suggestions and guesses based on using positive-resist, pre-sentized boards (Injectoral):
1) Sensitized boards have a limited shelf life. I store mine in a cool place, but after a year, there are very definite changes in terms of decreased exposure
sensitivity. Resist removal is more difficult probably as a result of that. Water will tend to bead up on the resist and wet the copper. So, after development, I look at the board carefully under running water to see if all of the resist that should have been removed was removed. If not, I will try more dips in the developer or even increase the amount of NaOH or KOH very carefully. If you cannot get the resist off, you probably have a lost board at that point. Increasing UV exposure time for another board might help. I do not think you can effectively re-expose a board that has been put into developer, although I have never tried it.
2) After it looks like the resist has been removed properly, I heat the board with an air blower or in the oven until the remaining resist becomes glossy. This step may seal very small pin holes. I think it gives a smoother edge to the traces.
3) I then place the board in ferric chloride etchant, and after a short time, check it to be sure the copper is etching. It will have a distinct appearance. If it is not etching, I rinse and try re-developiong it in a stronger developer. It is probably ruined anyway, so I am pretty liberal in adding additional NaOH. Then repeat step 2.
4) I aggitate the boards constantly in the etch tank and keep it at about 60 to 70 degrees C.
I have not had problems with boards up to the size you mention. The edges tend to etch faster than the center, but the constant aggitation helps a lot.
John