Sudden frame rate drop during rendering

I used the GUI to render some tiles across the screen with a sidescrolling offset.
Everything ran fine at about 80 FPS.

However about a few minutes in the framerate suddenly dropped to about 40 FPS and stayed there.

What’s really strange is that if I let my computer rest for a few minutes and run the program again the same thing happens (starts at 80FPS). If I relaunch it straight away the FPS is around 40.

Same number of draw calls, no randomly generated events, why is this?

Is there something I can change in the quality settings to rectify this problem?

Please, this issue is actually so severe to me it’s making me consider switching to XNA/Monogame.

I think it’s something to do with how much resources the game requests from the computer.
I noticed that games like Bastion would cause my computer to heat up faster (and my computer fan to work harder). Whilst running this game, barely causes the computer (windows 7 ASUS laptop) to make a sound.

I have a feeling it’s possible because another game made by Unity: Edge of space
Rendered just as much to the screen, caused my computer to work harder, without the infuriating framerate drop a few minutes later: