My 1.67 GHz Powerbook G4 with 1.5 GB of RAM runs Leopard without any problems. It might be very slightly slower than Leopard, but I like Leopard better anyway. It sounds like your computer is having a different problem. You’d probably get more results from following pascal_cuoq’s advice above, then maxing out the RAM like simpleD suggests.
Doing any or all of the following will likely be more helpful than going back to Tiger:
1. Check for programs loading at login under Apple Menu > System Preferences > Accounts > Login Items. Delete everything but iTunesHelper. (Note that unchecking the check box next to the application will NOT prevent it from loading at startup.)
2. Make sure your hard drive has lots of free space (at least 10%).
3. Since your computer runs fine most of the time, it’s possible that one particular program is making it slow down. You can use Activity Monitor (in Applications > Utilities) to find out whether one program in particular is giving you trouble.
4. Make sure you’re quitting applications when you’re done with them. This may not apply to you, but many people leave Mac applications running all the time, because they’re used to just closing the window on Windows. On the Mac, those applications remain active unless you explicitly quit by pressing command-Q or choosing Quit from the file menu. You can see the active applications by holding command-tab or looking in the dock.
5. Back up your stuff, reformat your hard drive completely, then reinstall Leopard. Add back only the programs that you need.
6. Add more RAM. Ideally, you should have 2 GB of RAM.