My unprofessional opinion is that there might be some serious problems. The reason I say this is because I had issues with a 6400. I received a new head from Canon under warranty, and it didn't solve the problem. I didn't have an entire color channel missing, but I did have many nozzles that just didn't look right. A tech was sent out to replace the carriage, along with new print heads. That didn't solve it. Then on another visit, the main board was replaced, and that didn't solve it. In the end, both the main board, and the carriage, and 2 new heads is what it took to fix it. The problem is that bad parts downstream can ruin the parts up stream. If the main board is ultimately bad, it will blow the carriage, which will blow the heads. So even when you put in carriage, it will be blown. Then even if you replace the main board, you've already got other bad parts down stream.
Now I'm not saying this is what is wrong with yours, but the fact that it just totally cuts out like this isn't usually a sign of blocked nozzles. If a new head doesn't work, then be prepared for a headache. These techs really can't diagnose exactly what is wrong, so they just throw parts at it. If its under warranty, no problem. But if you are paying for these parts, its a whole different ball game. And like I say, if you replace the wrong parts, and its still not fixed, the root electrical problem might have just blown the new parts you just put in. I honestly would give up if a new head doesn't solve the problem because everything else will be thousands of dollars with no guarantee that it will fix the problem.