Discussion about mapmaking in Heroes V.

Guys, I made a mini-map, just wanted to see how it would turn out. I managed to get 2 towns and 2 heroes on it, but there's a glitch: for no reason at all, the graphics become blocky (heroes, resources, and textures become pixelated), and it might revert back to normal, or it might stay like that for a long time. Basically, the graphics only degrade on my map. What could be causing this?
In other words, the map starts with great graphics, but as soon as I enter an unknown area, everything gets worse! And sometimes the graphics recover... it's definitely not a hardware issue! There must be some kind of graphics setting in the editor, or something similar.
Can anyone help me out?