some other tips
Look at the end shank of the bit that goes into the collet. It should have a slight chamfer rather than a hard 90 degree edge. If not, run it on some sand paper or a stone to chamfer it. Now insert it into the collect all the way until it bottoms out. Make a mark on the shank for reference. Next, pull it up about 1/32" and tighten it down. The chamfer will allow easier removal of the bit and the mark will show you how far down you are. It's best not to fully bottom out the bit for easier removal. Just tap it down with a block to loosen it if it gets stuck. Make sure the shank and the collet are clean and oil free. Now, you are ready to rout.
Depending on the diameter of ther bit and the depth of cut, you may need to make 2 passes to avoid putting too much stress on the bit. Your depth stop should have 2 or 3 positions, so use the that feature before going full depth.
Finally, the use of a dull bit will contribute to overheating and a change in depth because the cutter is working too hard and is being forced rather than allowed to cut on it's own. Maybe a better quality bit is in order? If you have a diamond hone use the flat portion of the cutter on the hone to refresh the edge. You will see a new shine on the flat where the hone is working and removing the carbide. Hope this helps.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)