Last year I had 4 machines out with two Phoenix Cards each for 4 inputs per machine. We were fine in the US but when we sent the system out of the US, we had similar problems. I lost 2 card in 10 shows. On 4 occasions I had to uninstall/reinstall the Phoenix Drivers. On one occasion I saw glitchy capture while we were live. The next day I mounted an 8ch HD-SDI Opto ($£ etc) in the rack and didn't see the problems anymore.

I will note that on 60 shows, the servers were in video village.
On 40 shows they were spread back stage and on stage. It was on stage where (sound giggling) where I noticed more issues.

Little notes I've made with Phoenix experiences:

I have 14 cards. Some are single some are double input.

2007 Machines with x1900 and Phoenix cards: 4 Machines and have never seen a problem averaging 55-60 shows per machine.
2008 Machines with 8800s and Phoenix cards: 8 Machines, replaced 3 Phoenix Cards in 2 years. Seen issues similar to yours on a dozen different occasions. Specifically have seen issues on touring shows and not the one-off/corporate. Average of 140-160 shows per machine.
2009 Machines with 4870 and Phoenix cards: 4 Machines and swopped cards in 2 machines once. Seen issues similar but not as often. Average of 25-30 shows per machine.

I will note that more often on the 2007 and 2008 machines I have seen Buffer errors more often than anything else.