Warning: Dropped Frames in playback

Running FCP 4.5 with OS 10.4.2
System: PowerMac G5 dual 2.7 w/ 1.5 GB RAM

Recently upgraded system from PowerMac dual G4 where same version of FCP ran without a single glitch on OS 10.3.

Tried everything I could think of or has been suggested by Apple Support. Did erase and install of OS, backed up my project and am running tutorial media as test from FCP disk, pulled added gig of RAM, did software updates (including Pro patch for Tiger), ran 'repair disk permissions' from Disk Utility and other than doing clean install of FCP Production Suite have put nothing on the Hard Drive.
Doesn't matter how I configure RT, mirroring off, or any of the other pref fixes the dropped frame issue won't go away. (Happening with my media and the tutorial media so it's not the projects.)

(FYI... Apple took back first G5 because they were suspect that the drive wasn't spinning fast enough, after trying suggested fixes above, they replaced it with an exact unit. Had first machine a couple of weeks.)

I'm stumped and so are the Support people... any suggestions would be appreciated.

Thanks

Do you have FCP 4.5 I have heard numerous of my clients having issues with FCP 5 and Tiger. no explanation, Apple can't figure it out. I suggest trying Removing the 5 install and p-lists and installing 4.5

I'm using version 4.5.

I'm at a loss...

Thanks for the suggestion.