Change the DV and see how you get on. Like Luke says, the oem valves only tend to stay healthy for so long, so changing it at some point is more or less inevitable.
I'm on an OEM rev G. It's been on for 18 months, and still seems to be working perfectly. And that's with Stage 2 boost levels.
Only problem with going for the GFB straight off the bat is that if it doesn't cure the problems you're left with a little less budget for troubleshooting. Although it is fit and forget. Depends how you see it; if you're keeping the car for a while or have plans for more boost then it wouldn't be a bad idea to get the GFB.
Shark are quite well known. Mansfield based. I'm not sure if the maps are generic or custom, but it's certainly not some cowboy outfit if that's what you're concerned about. Went round Oulton with Ben (of Shark) in their track car a good few years ago. Good chap and knows the game.
If it's a rev C it's likely the original valve. Even though you can't see any faults it's fair to say it's an old part that won't have much life left in it. The fact your boost is sporadic doesn't shout DV to me. But it's one of those jobs you could do with getting out of the way for the sake of preventative maintenance, like the PCV and cam follower.
