You could try to take the suspect holder and use gaffer tape to hold it in place? This might help diagnose a leak at that point? We had an Omega 45D that would cause these streaks of light (density) on the neg that for a long time we thought was an agitation mark. Turned out that it was this teeny-weeny pinhole and sometimes it would be there & sometimes not depending on how the bellows was flexed. We had to get a new bellows for that Omega, but in the interim, we still used it by wrapping a darkcloth around the mid-section (bellows) and clamping it tight. It was a PIA to work with everyday, but it kept the film from being fogged. Could be you could do some tests like this, and try to rule out the bellows. The other test might be to take black gaffer's tape and tape the hell out of that holder--get it in place, pull the slide--darken the room--and pop a strobe into it at different angles a couple of times. Then run the film...

Could also be a developing error.... These things are really hard to track down and will almost drive you nuts....

KT