Tempeture in tank High on cool side

I'm going to move my boa's viv today - the temps have been so erratic. It's been unseasonably hot in the UK (100 today!!!). The viv is either too hot or too cold. I've got it under a window so am thinking of shifting it - it's 4 ft though ... :( will take a couple or more of us to do.

the tank is near a window but there is no light coming through, the window is inside of a porch that is inclosed with latus and vines growing on it with little lite getting through, the temp gets down at night and in to the perfect ranges. Like i said durring the day this room is the coolest room in the house! I checked the weather report and it looks like the next week or so wont be to bad alot of rain and over cast.

Its definetly not a equipment problem, the thermistat is working and so is the heat pad it gets turned of during the day when it gets to warm. I went out and got some more fans for the room and they are not helping much.

Im honestly thinking of moving to tubs or a custom built cage that i can instal vents in that can open and close for more or less air circulation these Critter cages are so much trouble.