We have around 50 hours on our -12 and have experienced this exact problem. In October we replaced the old style fuel pump with the new one (just as a pre-caution). About a month ago we did a takeoff and the engine went very rough right after adding take-off power and then could not replicate it again. Then a couple of weeks ago it was happening on the ground almost every time power was added. Did the "mag check" and found that it did not change anything. So we pulled the carbs and replaced all the suspect parts everyone has talked about here. We started it up and did lots of full power run ups and everything appeared to be great so we took off and flew around. I did notice the fuel pressure dropped a little when climbing, something I had not noticed before. So, over the airport (probably should have tried this on the ground first:roll eyes

I pulled the fuse of the electric fuel pump. I have done this many times and typically the fuel pressure drops a few psi and then comes right up. This time it dropped to .1 psi and the engine started missing. It came back to life after I put the fuse back in and we landed. On the ground I pulled the fuse again and it did the same thing. Went to about .1psi but this time the engine kept running and didn't go rough.
We have a new mechanical pump coming and will update everyone on what we find. We may check the float adjustment as well.
Everyone may want to try this test on the ground and see if there was a bad batch of the new style fuel pumps out there.
The other thing we cannot figure out is the fuel smell on takeoff during this time. We never had it until these past few flights and can sometimes see a little fuel in the aluminum catch under the carbs. If the mechanical pump is bad, why would that cause fuel to come out of the carb. It does not look like anything has come out of the overflow tube on the mechanical pump.
I will update after the new pump is added and more checks are carried out.