1) Portals should be as close to background color as possible and as light as possible. 2) Set portal background to colored (same as background and not transparent, 3) Same with lines, ie, keep their color light and keep lines at smallex px width possible because the thicker the line, the more it flashes, 4) Select the portal and Arrange > Send To Back. Fields are always on top of a portal but if they are earlier in stacking order, there will be additional flash as FM renders, 5) Make fields within portal high enough to easily show one line of text, 6) color field backgrounds (again no transparency), 7) keep fields away from edges of portal, 8) if you have text labels above the portal, select them and Arrange > Send to back so the portal does not have to render first then again AFTER the text has drawn, 9) try to keep unstored and aggregate calculations to a minimum in portals because they must re-draw often and 10) try to keep the entire layout lean of objects (don't overload the page) and keep all objects away from each other and be sure their stacking order is efficient.
As you do each thing, you will notice subtle (and no so subtle) differences as you then scroll through records. Do one thing at a time and then try the result ... it will show you alot about the flash effect.