Bash Terminal keeps clearing / overwriting itself

Question

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