Slow screen display, sonar 8.5.3, after machine transplant

Over the past week I had to migrate my whole system to another machine, due to a persistent motherboard problem. It was a pretty lateral move, from one old XP SR3 machine to another, both Intel P4 hyperthreaded (but no multi core), both 4 Gig of Ram and both fast 7200 RPM SATA HDs. The CPU on the new machine is marginally faster, 3.2 Ghz instead of 3.0. Anyway, as sonar is my most CPU intensive application, I was pretty happy to see the same project that required over 50% CPU usage to drop to less than 30% on the new one. Yet when I PLAY a project, despite there being no problems whatsoever in the sound or any problems with CPU or disk usage, the NOW time marker and the whole user interface now staggers along. All the screen updates are now sluggish, as are responses to my mouse click commands. This is going to be a problem when I try to manually punch in as I often do when touching up vocal tracks. At first glance, i thought surely this is a video card issue. My old machine had its own separate video card, while the new one is currently using motherboard video. I have a video card on order, but I'm skeptical. I'd think if Sonar were having trouble keeping the screen and UI updated, the CPU meter would be showing the system straining. Not so. I've tried to select Sonar in my XP task manager and boost its priority, but nothing changes. Any ideas?