Well here is my experience. I am running a i5 8GB Ram machine on windows 7 64bit. (these are as of my connection being 24Mbit/2Mbit. I just got my 50/10 recently, but I haven't retested, although I feel it will be the same or similar.)
When harvesting I typically don't push it above 75-125 connections. 75 seems to be optimal for me. Similar in posting/link checking etc... 100 connections.
There seems to be a threshold for ideal performance. I can run 3 instances at 75 connections harvesting and get quicker more stable results then 1 instance at 150. I can run 5 instances posting or link checking at 100 connections with much better results then I can run 1 connection at 200 connections. I get way more timeout errors an other errors.
Point being I turn connections down and increase my instances and get better results. Depends on your system configuration, I just tested different settings and settled on what I use now.
Also it works with sessions when harvesting. So Google for instance sees your IP, but they also see your scrapebox instance as a separate session from other scrapebox instances running on your PC. So each SB instance gets its own treatment in addition to your IP getting treatment rules.
So keeping your connections down will help your particular session/instance will help minimize your issues. (this is from my understanding of it, which is not rock solid, so someone correct me if I am wrong).
But even when unblocking ips via the proxy harvester for a particular SB instance with Google doesn't unblock them for the other instances.
The short version there is that Greg is on point, the first thing I would do is try turning your connections down, like real low and see what happens and then slowly increase them and see where your cut off point is. If that doesn't work then go from there.