Need some more info on whether this is a general problem, and whether you're using the integrated trackpad here, and/or any external USB hardware. Read the whole reply here through, before trying anything - some of what I'm suggesting will not apply to your particular case.
Is this problem effecting other applications in general? Does this positioning problem effect only Mail.app, or is this jumpiness problem arising with other applications?
If it's a general problem, see if you're getting bad positioning data from the trackpad or the keyboard. If you have flaky hardware.
If you're using any external USB devices here (at all), remove them and test again. If the problem goes away and when you're using an external mouse, remove the external mouse, then test swap it for a different mouse.
If you're using the integrated trackpad and not external USB hardware, then first clean the keyboard and trackpad (carefully following Apple's suggestions for that), and then I'd try testing with an external USB keyboard, and an external USB trackpad or mouse.
As I've seen it in a few cases, I might also disable the internal trackpad, as a test. I've worked with a few folks that were brushing the trackpad. It is possible to disable the trackpad entirely - for testing, here - on recent releases, but I don't recall if this capability existed as far back as OS X 10.5. In OS X 10.7, these trackpad settings are in the Universal Access control panel in System Preferences.
If it's just Mail.app that's involved with the wonky behavior, then first try creating and using a new user and see if the problem is local to your own preferences. If that (also) shows the same problem, then I'd see if downloading and re-installing the 10.5.8 combo kit - the combo kit, and not the delta kit - corrects the problem.
Thanks for your suggestions. I've tried several tests and determined it's not my extermal trackball or the integrated trackpad. It's definitely the integrated keyboard. I'm typing this now on an external keyboard with no problem. Attempting the same with the integrated keyboard is virtually impossible.
I couldn't find anything in Apple Help re proper way to clean the keyboard on my MacBook.The surface looks pretty clean, however. Do you know if there is anything that can be done to fix a defective integrated keyboard on a 4 year old MacBook? Thanks again for your suggestions.
If this misbehavior isn't focused on a specific key or a specific area on the keyboard, and if there wasn't a liquid spill, then it's probably not crud.
It's probably a problem with the keyboard itself, the keyboard electronics, or the a cable.
Since it occasionally comes up in this context, I would not expect that resetting the SMC would help here.
Check with the local Apple Genius or Apple resellers, and see what they think about this. And see whether or not this is an expensive swap.