Bash Terminal keeps clearing / overwriting itself

Bash Terminal keeps clearing / overwriting itself

My bash terminal on Ubuntu 12.04 64 bit keeps clearing itself partially
when the output is too big.
If I do a ls -al in a directory with many files. The terminal prints the
result, inbetween it scrolls down without printing anything and then
continues with the remaining result.
Or when I am tailing a log file I often see the output is overwritten in
the terminal. If i scroll up I can see parts of output replaced by blank
space.
Note: I have kept scrolling in Profile Preferences for terminal as
'Unlimited'.I never had any issue before of this kind