Right now, I have MyHD, Fusion, and HDHomeRun tuners. MyHD is my favorite playback device into my HDTV because it produces a great picture from its independent output. However, as I mentioned above, it is not a great card for desktop viewing. The overlay output is only 480p. The overlay was really meant to be more of a monitor window, not the primary way to view output from MyHD.
I would only recommend the HDHomeRun if you you are using a front-end, such as Windows MCE, Sage TV, Beyond TV, etc. Out of the box, it will work with VLC, but I am not a big fan of VLC. It is a good value since it is a dual-tuner and it does what it is designed to do very well. Also, it does not do analog.
Historically, the Fusion cards have had buggy software, but DVICO has done a great job improving it over the last 18 months. DVICO offers PCI, PCI-Express, and USB versions. In view of the problems you are having now, I would recommend the USB version, which will isolate the tuner from your system more than the PCI or PCI-Express versions.
I don't have an AutumnWave OnAir GT, but this tuner might be a great option for you. It is USB, and it has an LG 5th gen tuner. It also comes with the nVidia PureVideo Decoder. I have TheaterTek, which also uses the PureVideo Decoder. In my opinion, TheaterTek looks better than Fusion when playing HD TP files, so the OnAir GT might look better than the Fusion. BTW, I still like the MyHD output a bit more than TheaterTek for HD files, but I prefer TheaterTek for DVDs.
If you are considering either the OnAir GT or the Fusion, you might want to post in the appropriate thread and see if anyone is also using a 2600XT.
Finally, if you have dual core or hyperthreading, it can't hurt to run with a single CPU and see if that makes a difference. Take a look at the Meta FAQ sticky.