Mash temperature in all-grain brewing has a significant and demonstrable affect on beer flavor. In general, multi-temperature mash profiles incorporating temperature rests from 120* to 160* F will produce more fermentable worts than single temperature mashes held between 152* and 154* F. In flavor terms, beer produced from highly fermentable wort tends to be drier, crisper and perhaps lighter in body than those made from less fermentable worts. Another key difference is that alcohol content goes up with wort fermentability. I could go on and on about the “technical” differences in these sorts of beers and make an excellent argument for the importance of very precise temperature control. Indeed, these concerns are very real to commercial brewers because temperature control and the ability to repeat mash temperatures directly affect beer consistency. And consistency is one of the most important keys to commercial quality.
When homebrewers think of quality, we think of the sensory qualities of the beer. Does it taste, look and smell like intended on brew day? If it does, then all is great and we have a quality brew. However, if you only want to brew that one beer, then you should attempt to be as consistent as possible with your mash temperature, along with every other key variable in the brewing process. On the other hand, if you want to brew a wide assortment of great tasting homebrewed beer, I would not get overly concerned with your particular problem. It sounds to me that the mash stays in the desired range for most of the time and when it does cool off you heat it up in a reasonable time frame. I would be more concerned if you were getting the mash too hot and trying to cool it down because that destroys enzymes.
One easy solution to your problem is to insulate your mash tun. It sounds like you have a stainless-steel mash tun heated by a propane flame. You don’t want to insulate the bottom because the vessel would be difficult to heat if the insulation was doing its job and the insulation might burn (unless it is resistant to flame, like rock wool, glass fiber or, asbestos). If this were my project I would buy a non-flammable insulation material and insulate the sides of the mash tun and make some type of simple insulated, lid to prevent heat loss from the top. You could probably find these types of materials in your Air Force locker!