I have had kinda similar problem with just a .Picture and BMP;
maybe one difference (and that's where you should look into) : when a long time it worked, suddenly it didn't anymore.

I've written many pages on this subject, trying to reason the how and why, but couldn't come to a solution. When these pages were finished, a few days later all suddenly was okay again;
My pages had one definite conclusion : VFP doesn't respect the number of colours which it should. This by itself can still be shown today :

When 256 colors are installed for the monitor (etc.) there exist 18 (not 16 !) plain colors. VFP shows less on pictures (it's hard to see how many, but for sure less). I am not even sure wehther the plain colors have to do with it, but for sure it's not 256. I mean :
When creating a Form with its colors everything is okay and meets the 256. But as soon as you throw images (pictures) far less colors are accepted.
I think I can tell some more for this matter, but don't have it in mind right now. Anyhow, I think this is your problem, or anyhow the basis of it.
This momentary time a bmp showed wrongly, it got disturbed with horizontal (black) lines.

Tip : wait till it gets away ?? {g}