NL50 top pair gets reraised oop

[BadSeed]

A fairly typical situation there. Assume there is no observations yet on the villain.

Hero - 50$.
Villain - 50$.

Hero (SB) A [img]/images/graemlins/spade.gif[/img] J [img]/images/graemlins/spade.gif[/img]
CO calls 0.50
Hero bets 2.25
Villain (BB) calls, CO folds.

Flop (5.5$) 3 [img]/images/graemlins/diamond.gif[/img] 9 [img]/images/graemlins/heart.gif[/img] J [img]/images/graemlins/club.gif[/img]

Hero bets 4$, Villain raises to 8$.
What is our line here?

Apply the same situation to two different flops:

A [img]/images/graemlins/club.gif[/img] 7 [img]/images/graemlins/heart.gif[/img] 9 [img]/images/graemlins/heart.gif[/img]

A [img]/images/graemlins/club.gif[/img] 7 [img]/images/graemlins/heart.gif[/img] 3 [img]/images/graemlins/diamond.gif[/img]

My original thoughts were that in the first case scenario any Jack and some pocket pairs will reraise us flat just to see where they stand, but what conclusion can I drive? Reraise to 20$ and fold to further action? Or call, and check/call reasonable bet(s)? In the second case scenario I am inclined to defend my pair against draws, but in the third case I am worried enough about stronger hands and weak kicker to consider call, turn c/f.

What do you think? This is one of the most frequent and annoying and -$ situations I am facing with a decent hand...