Well, as far as this forum goes, it's clearly a switch problem ;)
In all seriousness, there probably isn't enough information to go on here. I have swapped out Gig interfaces in a 535 several times with no obvious issues. I would suggest that you have 6.3(x) on your PIX as we had some "issues" in pre 6.3 code figuring out how to perforn Gig negotiation of the link. This could have been part of the issue in the losing your link issue.
Any more info you can provide will help.
Scott