Thanks again everyone for your help.
I tried a few different things and I've finally had success.
No matter what I tried, the screws would sink through the plaster. I got to thinking about how sheetrock works. It's plaster incased in paper. So wherever I knew I had to put screws, I actually ran a span of joint tape & compound. As it dried, I sunk the screws in by hand with a screw driver and it worked perfectly!
The house is coming along great, but now we're considering selling this summer.
Anyway, thanks again!
Rev