Thanks for all the info, Folks.
I decided to give YOSEMITE 10.10 another try. (Third time lucky? . . . or just plain stubborn.)
I had done the drive scan. No alarms reported there. Replaced the two 8-GB RAM sticks with two 2-GB sticks. Same startup problem, so put the 8's back in again. (Yes, both sets of sticks - or the RAM slots themselves - could have issues, but checking the System Report seemed to indicate that all sticks registered correctly when snapped into place.)
Then, I did the following (i.e. clean install) :
1. backed up all my stuff on the EXHD using Time Machine
2. ran "one-pass ZEROS" to erase the drive using Apple's ONLINE recovery/DU
3. ran "repair drive" using same DU
4. installed Yosemite.
The whole process took the usual amount of time and loaded, installed, re-booted, et al, as expected. No stalls, hangs, freezes, blank screens, crashes, or the like. So far, so good.
However, I will NOT transfer my stuff (yet) from the EXHD, but instead stay with the MBPro/10.10 factory default settings, etc. See if any "start up" issues develop. If all seems OK in a few days, I will then use
Migration Assistant to transfer my goods and settings from the EXHD.
I have always used MA to do such transfers, but is there something else I should do which would reduce the possibilities of glitches along the way?
I was thinking of adding the applications one at a time. I only have a dozen or so that did not come with Yosemite. A slow, PITA process for sure, but not too daunting. Perhaps there is a third-party app which has been the culprit all along (????). Just guessing here.
Again, Guys, thanks. Will update either way : With cheers all around . . . or frisbee the gizmo out the window!
P.S. Early 2011 - 17" MBPro with Yosemite, i7 core and 7200 rpm HD.