I agree.I have used poplar in the past,found no knots,sands very easy,paint takes to it like a charm.The thinner wood BB ply unless you have a plainer.
I can understand your wood problem?I had the same run in with Cedar a few years back,1 board was a breeze to cut,the next was like cutting thru a cement block.Hope you get it figured out.
Kevin,I have never told anyone to get plastered,(I'm not a Drinking man)But in Your Case,get Stoned for a night!!Then wake up somewhat fresh ,maybe, in the next few days.