I have a 600 dp that had exactly the same problem. Sometimes I would even get an occasional unexplained backfire through the carb. This was the result of unburned fuel in the manifold. I got the carb (all milled) for a good price because the P/O couldn't make it work right. Like most people thinking bigger is better, he actually put in a 28 squirter!
For some reason the 600s are very sensitive to the pull from the venturi and the pump shot can make them go way rich, causing the bog. Most people think they go lean, but I did A/F on the dyno a few times and can confirm that it is a rich condition. CarCraft Magazine found the same thing on their 600 dp and filled and drilled the 25 squirters to 22s before realizing that Holley makes 21s.
I've been thinking of assembling a kit that would allow people to actually measure the pump shot as there is nothing like that offered on the market.
Try to reduce the squirters from the stock 25 to 21. Also put the secondary pink cam on the #2 position. While the #2 position is SUPPOSED to give the bigger shot, my testing found this is not the case and #1 provided a larger shot. In fact, I found that all of the published values are way off and the cam is very sensitive to even slight variations in placement
I run Primary - 69, Secondary – 74, Squirters - 21
When I had the primary at 67 I was just getting a slight lean surge at light cruise. The 69 took care of that.
This shows how the A/F goes to ~10:1 where the engine bogs from the squirters and power recovers as the A/F leans out.