Help needed to understand erratic behaviour (memory management?)

Hi all!

I have started developing a racing game on my Gamebuino Meta, and I have been encountering lots of random runtime issues as my game grew bigger.

To describe quickly my game, it's a racing game with an Outrun-like view, so I need buffers to store the depth of each line, the curvature of the road (turns & hills), positions of sprites, etc. I am using the full TFT resolution, with 16-bit colours, so I am very cautious on my implementation. Of course, graphics are generated on the fly and fed through "DMA" accesses to cope with the lack of a big frame buffer.

Everything was fine at first, everything was stored in big static buffers, and my first prototypes were running smoothly. As I started adding features, strange stuff started to happen: my color palettes began to shift, after which one of the LEDs started blinking at the same rate as my palette swapped (funny, hey?), and file reading through the SD library was limited to 256 bytes (read() returned -1 as soon as I started reading the 257th byte).

gb.getFreeRam() return ~7kB, so that does not look like a collision between the stack and the heap.

I started suspecting a side effect of memory fragmentation, so I started implementing my own memory management, in an 8kB global buffer. And everything came back to normal. Until today, I am starting to add sprites, and my palette restarts shifting. I still have 700 bytes in my custom buffer, 6kB in gb.getFreeRam(), only instances of PODs (plain old data = base types) on the stack. 

Sorry for the fuzzy description, but I don't understand at all what can happen. I suspect I am hitting some limit (hard? soft?), and I cannot continue my game (I can display 1 sprite, and if I have 2 of them, none appear). I am quite knowledgeable of C++ and software development, but not on embedded platforms, and I suspect I lack a bit of experience on this matter.

Any explanation, suggestion of test or anything would be greatly appreciated :-)

Update: I was wondering if some addresses were reserved and I could be hitting them by mistake, thus explaining why my palette and the LEDs were altered simultaneously