I avoid themes that use images as their background. Also, check and make sure that there are no visible or hidden objects that overlap your layout part boundaries. It looks like your white subsummary titles might be overlapping. In Layout mode, do a ctrl + A to select all and see if anything looks like it's overlapping a boundary line.
Also, what OS and browser are you using? Have you tried it in another browser? Does it also happen consistently there?
I deleted the labels (which aren't in the subsummary section but are in the Leading Grand Summary section above the subsummary section. No change in the visual glitch.
I'm using Windows and Chrome. Also tested IE and that has the same issue.
I changed the theme to Enlightened and that did not have any effect either.
It's still rather odd that it only affects some records and not others, and it can be duplicated. Is there anything notably different about that record? EG special characters in the text? multiple rows worth of text?
FYI, UI stuff doesn't really matter if it works in pro/go, webdirect uses a completely different method to draw what you see in the browser.
Without having access to your solution and the limited screenshot, I wouldn't be able to really speculate further as to what is happening. You may want to make sure that you have updated to the latest version of filemaker server. If you have time you might want to try recreating the layout as well. If you want to share access with me privately, I can take a look to see if I can spot anything else obvious, just send me a message.