Update to 5.03 and see if that resolves it.
Last Night I discovered one moe thing. If you play the target video clip (space bar) prior to dragging the tif file overlay the probelm goes away. Once that one is keyed on the clip, if you scrub down-range to another video clip and don't play it first, the problem always crops up again. So there is a workaround. It may sound a little superstitious, but I always hesitate to update in the middle of a 90-minute project.
I've owned every version of PPro since v.4.0 (1997) and this is the first version I've noticed this behaviour. I'm still wondering if it's just here, or perhaps others have seen this.Or maybe some obscure setting of some type is at work.
Open and resave the images in Photoshop, and then try again. If that works, then you're encountering a bug that has been fixed in Premiere Pro CS5.5.
Regarding the updates: There are a lot of bug fixes in the 5.0.2 and 5.0.3 updates. I strongly recommend that you install them.
If 5.03 is just like 5.01 in behaviour with just some bug fixes I think I will.
But just a note: I tried 5.5 and I didn't like how it handled the red/green line render-issue. It was always all yellow and seemed slower than 5.01. So I backed it out and have been mostly happy since. It fits my workflow. The Mecury render business seems like a waste of the 800 bucks for the video graphics card I used too. I'm glad it wasn't just me noticing some little bugs in 5.01
One other thing 5.03 might fix too. Sometimes if I take a long break, and come back to work, the program seems to go to sleep. When I click on a few things some of the panels go all white. Would that be a bug too? At first I thouhgt it was the external HD I use fro each new project. Maybe not?
Panels going white is a Windows issue, not PR.
For Panels going white, I would look at your video driver. Go to the nVidia (or ATI/AMD) Web site, and check for a newer driver. If there is one, download and install that.