advice on hitting mash temp with biab?

So I just did my first all grain batch yesterday using the biab method. I had a quite messy brew day (pot was too small, my hydrometer broke when i tried to take a gravity reading) but my biggest problem was inconsistent temperature. I was mashing with the lid in an aluminum pot. i had quite a bit of loss of temp, it went down to 140 for a short amount of time, and when i either put the heat on very very low for a minute or two or added a quart of pretty hot water (tried both) it shot up to over 160. I tried to keep it as consistent as possible, how can i fix this problem in the future?

First, you could insulate your kettle: put a jacket/coat or some other form of insulation around your kettle.

Second, there's always a good chance that the temp in your grain bed will be lower than the liquid surrounding the grains. While heating, make sure you stir every few minutes to keep it even (assuming you're heating your grains and your water together), and also stir a few times during your mash.

Also, do you heat your grain and ALL of your water together? That will help a bit, but if that's not what you do, and your kettle is already too small, it might be the time to upgrade to a larger one.

if you use a small kettle, like 5 gallons in size, pre-heat the oven to its lowest setting, then put the kettle in there with the heat turned off. it'll hold within a couple degrees. If you have a larger pot, wrap it with something to insulate it. I use an old sleeping bag with my 10 gallon kettle and I'm making a removable insulating jacket for my keggle. this normally keeps me within a couple degrees over a 60 minute mash.

If you do have to heat during the mash, make sure you stir it non-stop while heating. this will help keep the temps closer to equal and let you see when you hit the temp you want.