On the 'mash' tab, are you entering the temperature of the grains and of your equipment? This is needed to calculate the correct heat transfer to bring these in balance with the temperature of the water. The strike temperature of the water must include the energy needed to raise the temperature of the grain and equipment if you are having the program adjust the water for the equipment. If you are seeing such a swing from summer to winter, it would seem that you have not been updating these temperatures on the mash tab.
When you say you are losing about 5 degrees, am I correct in guessing that if you want the infusion temperature to be 154 the temperature of the mash once you have added the water to the mash tun and stirred in your grains is actually coming out to around 149? We are talking about initial infusion temperature and not the temperature loss over the time of mashing, aren't we?
If I am correct about the above, then the next steps is to adjust the specific heat of your mash tun to get the temperatures to match your actual results. You can do this by making a copy of a recipe you brewed, edit the mash profile within the recipe to match your actual temperature achieved. Once you have done that, edit your equipment profile within the recipe and change the specific heat for your mash tun up (if you are low in temperature versus your target) or down (if you are higher in temperature vs your target), until the strike temperature calculated by BeerSmith matches the original calculated by BeerSmith. Once you have reached this, or close to this value, then save the equipment profile by clicking on the disk icon from within the recipe. Now when you update the mash profile within your recipe copy back to your originally targeted mash temperature, the program should give you a corrected strike temperature which will bring you closer.