The Sonnet hard-starting can be due to a number of factors.
The most likely factor is that your pen still has 'factory' oils in the feed. Flushing out your Sonnet with a solution of dish soap water may help flow improve.
I have used Noodler's Zhivago (bulletproof - similar to noodler black) in my Sonnet, and it works just fine. I prefer Quink black in it though because it's wetter.
If your pen does not improve after flushing it out or trying different inks, I suppose you could seal the vents in the cap. There are many ways to do this, but the way I use is melting some candle wax in small tin cap, deep enough to dip the top of a Sonnet cap (so that just the bit of the clip that joins onto the cap is submerged). After pulling it out of the hot wax, I would do some squishing and wiping to remove the excess wax. I can check for airtight by putting the waxed part of the cap in a bowl of water and blowing (air bubbles will leak out if you have gaps.
This process may take a bit of trial and error, and isn't the easiest thing to do in the world :doh: If you still have problems - it might be best to try to replace the whole nib, as it may even be a problem with the nib. Sonnets are a bit of a gamble due to Quality control issues - so look out for dodgey nibs.
good luck