I am guessing you are meaning your temps creep up after switch on and don't seem to stabilise, rather than a more long term temp creep due to losing performance over time, e.g. by some form of progessive blockage. In which case then yeah more info is needed, however one main factor is going to be how your rad is mounted in terms of its air flow. If it is mounted such that it draws its air out of the case then temps are going to depend on the ambient air temp to the rad, i.e. case heated air, which depending on your in/out fans set up in your case can increase from switch on and therefore possibly make the time to reach an equalised temp seem so long that you think it won't stop creeping up, as well as generally increasing whatever the stable temp would be.