I had that problem too. Then, on the advice of dozens of members of this forum, I quit putting any liquid in the water pan. I've had no problems since.
In case you haven't read all the posts about the water pan in the MES, the bottom line is that the MES has an air turnover (how often air moves through it) of about 1% of a more traditional smoking apparatus. In other words, the smoke and air sit there for a long time. You can prove this to yourself by putting your hand over the open vent. What you will feel is a very small amount of warm air. If the air was moving through more rapidly, it would feel really hot to the touch, and you have some sensation of "wind."
The problem with low air flow is that the moisture from the meat builds up inside the smoker. With all that moisture, you don't really need (or want) to add more moisture with the pan. The moisture from the meat will provide enough moisture.
Some people fill the pan with sand in order to have it distribute the heat a little more evenly.
And, speaking of heat distribution, one other problem with the earlier MES designs is that the heating element was on the same side as the vent, creating a hot column of air on the right side of the smoker, while leaving the left side relatively cooler. Again, you can probably see this yourself, if your smoker has that design, because the cooking shelves will often have food residue that is much harder to remove from the right side (over the heat) than the left side. That is because it has been baked on. The bottom shelf is often worse than the upper shelves, for the same reason. Unless I need all four shelves, I don't put food on the bottom shelf.
