Weird video on Apple IIc

[macrl@pantheon.yale.edu (Ryan L Macarthur)]

      I called 1-800-SOS-APPLE on this, and was referred back to Nathan 
Mates's home page,"If you can't find the answer there, I don't know 
where else you could," the tech support guy said when I told him it's  
where I started from.  So perhaps he, or any of the Apple II faithful 
could help me out here with telling me what's wrong with my //c.
     I recently got my hands on an old Apple IIc [? PEEK(64447) ==> 255] and 
it seems to have a couple of problems, the most disturbing of which is 
the video display.  It's the normal "Apple //c" screen when the power's 
turned on, but there are a couple of rapidly blinking @ symbols staying 
in the same place on the screen.  After the disk drive reads the first 
track or two on any disk I put in and crashes out (for example, the ProDOS 
1.0.1 or "David-DOS" header will appear on the screen, and then the drive 
starts screaming bloody murder... I'm thinking maybe it's the drive, 
maybe it's another symptom of a deeper problem, I can't tell until I hook 
up an external drive and try booting off that), the symbols are still there, 
and when I hit CR the @ symbols start forming a column up the screen:

               @
               @
        @      @
        @     
        @      @
        @      *
        @
        *

where the *'s are the rapidly flashing @ signs, and the ones that scroll 
up the screen are normal.  Applesoft BASIC seems to work fine, and as the 
screen is filled with text, text that is scrolled up past the flashing 
@'s will be changed to a different character.

"Sounds like some weird memory problem or something," 1800SOSAPPL Apple 
II Tech Support Guy says.
"So I guess the only solution is to try and get my hands on a new 
motherboard or something," I says
"Or buy a Mac," the Guy says

    When I switch over to HGR, there are about a half a dozen pixels on 
the screen which appear in the same place every time (as do the @'s), and 
a little program written to draw 10 random lines on the screen, refresh, 
and repeat works fine but doesn't in HGR2, but maybe I don't understand 
Applesoft BASIC that well.  Lines will draw over the pixels on the screen 
(which also blink a little) and will leave the pixels unaffected (a white 
line over the pixel has a black spot on it, and the pixels will remain after 
CLEAR).
    When the keyboard is pulled off after power-up (it goes into 
self-test otherwise), the problem is still there, which brings me to my 
final question: what am I supposed to expect from the Apple //c self-test?

Any insight would be whole-heartedly appreciated.

-Ryan