Fordmanf150

Have a question I'm new to this site, i just built me a smoke house, and have had trouble keeping the temp the same thought out the smoke house even on the same rows the temp read one thing on the left and something different on the right. For about two hours my suasages seem to cook well but after that they seems to stop cooking, get soft and start to wrinkle up real bad have any one had this problem with there smoke house, do i need to insulate the inside of it or add some planks to hold the heat in it more please advise.

Thanks
Fordmanf150