I’ve noticed this now on enough systems to call this a bug, and since it involves 2 out of date pieces of software and one out of date graphics card, it’s unlikely that Apple will address it.
Problem: Skipping playback in the Canvas and/or Viewer windows in Final Cut Pro 7 on Mac Pros with a GT 120 graphics card running Mac OS X 10.7.5. I don’t know if this problem exists on Mountain Lion as every system I’ve come across with this issue has been on 10.7.4 or 10.7.5.
Solution: If you must use Final Cut Pro 7, the only solution that I have found is to get a new graphics card. Some have reported that the problem goes away after a restart, but eventually comes back. I have an ATI Radeon 5770 and nVidia GeForce GTX570 (flashed by macvidcards) and neither one of these cards exhibits this problem. I can actively replicate this bug, as I keep both the 570 and the GT120 in my Mac Pro for DaVinci Resolve. If I open up Final Cut Pro with the monitors connected to the GT120, I get skippy playback. If I instead connect the monitors to the GTX570, playback is smooth again. I’ve also noticed that Safari and Chrome scroll like crap when plugged into the GT120. So I only hook my monitors up to the GT120 when I’m using Resolve. Otherwise, they’re connected to the GTX570.
A friend called me today with this issue. He’s still on Final Cut Pro 7, and we discussed the reality of switching to Premiere Pro. He doesn’t have to switch for his current projects, but would like to learn Premiere and move forward. So I suggested he get the GTX570 from Macvidcards so he can take advantage of the CUDA processing once he switches. The ATI 5770 is cheaper (I have one for the occasional Apple Color project), but I advised against it as it doesn’t make sense moving forward – FCP can’t tell the difference between the cards, but Premiere Pro can take advantage. So the GTX570 has a bit of future proofing. As much as fast changing technology can have anyway.
One thing to remember if you’re looking at new cards for a Mac Pro. The only Apple blessed cards are the ATI Radeon 5770 and 5870, the nVidia GTX 285, Quadro 4000, FX 4800, and GT120. These are all incredibly old by PC standards. Macvidcards will flash nVidia cards with new ROMs to make them Mac compatibile. With the GTX570, this means installing the current driver (nVidia keeps the Mac drivers up to date very well) and you’re good to go. But if you wipe your hard drive clean and install a new OS, your card will no longer work. You will need to reinstall your GT120 or other Apple-blessed card to see what you’re doing, download the newest nVidia driver, and then swap cards again. Not a huge deal, but something you should know.
BTW, Macvidcards’ website has very little info. Go to their eBay store instead. Search for “Mac GTX570” on eBay and click on the first card in the list and it likely will be from seller Macvidcards. They sell many cards in addition to the GTX570. I like the 570 because it’s the most powerful card you can put in a Mac Pro without needing to beef up the power supply. If you’re using an external expansion chassis like the Cubix, go with the GTX580 or perhaps the new Tesla cards (I haven’t tested this though, so your mileage may vary).