Last night I was running one my IHC 2-6-0 camelbacks with a short train of five 40’ boxcars and a caboose. Each boxcar weighs the NRMA recommended 4 ounces and the caboose is 3 ½ ounces. At first the locomotive performed flawlessly. Then suddenly it began to stutter and have difficultly pulling the train. The layout is level. There are no grades. After a few moments the locomotive stopped completely. There was no hum from the motor. With some coaxing I could get it to run for a short distance. I removed the cars form the locomotive and it ran with some stuttering but then it ran fine. At this point I cleaned the track and the locomotive wheels. I placed the locomotive at the front of the train. For two laps all was fine but then the locomotive started stuttering again. Halfway through the fourth lap it stopped. I am now thinking something is up with the cars in the train. I remove them and the loco runs fine. I begin adding the cars one at a time. One car fine, two cars everything is still OK. I add the third car and the loco starts having difficulties. I remove the third car and replace it with another with the same result. When I add the fourth car the loco stops. I then bring out my other IHC 2-6-0 camelback thinking that it maybe a problem with the models. I hook up the 6 car train and the second locomotive has no problems. I run it around the layout about 20 times with no noticeable loss of power. After racking my brain for a while I notice that the box that the locomotive was stored in has some damage to it that I had not noticed before. I am beginning to wonder if it was dropped or knocked off the layout. I examine the locomotive and cannot see any signs of damage. From what I can tell the drivers are in quarter. When the locomotive is running fine there is no shaking or swaying. When it starts to stutter the headlight dims and brightens accordingly. Any suggestions as to what to check next.