NL50: AK vs unknown, sudden turn agression.
 

Villain is UTG+1 and he is unknown again - 16/17 hands.

PokerStars No-Limit Hold'em, $0.50 BB (8 handed) Poker Stars Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

Button ($31.85)
SB ($102.10)
BB ($28.55)
Hero ($64.55)
UTG+1 ($73.60)
MP1 ($25.65)
MP2 ($22)
CO ($51.25)

Preflop: Hero is UTG with K[img]/images/graemlins/diamond.gif[/img], A[img]/images/graemlins/diamond.gif[/img].
<font color="#CC3333">Hero raises to $2</font>, UTG+1 calls $2, <font color="#666666">6 folds</font>.

Flop: ($4.75) 7[img]/images/graemlins/club.gif[/img], 6[img]/images/graemlins/heart.gif[/img], A[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $3.5</font>, UTG+1 calls $3.50.

Turn: ($11.75) J[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $8</font>, <font color="#CC3333">UTG+1 raises to $18</font>, Hero ?

Re: NL50: AK vs unknown, sudden turn agression.
 

Fold. I usually c/c the turn here.

Re: NL50: AK vs unknown, sudden turn agression.
 

Fold it!

Re: NL50: AK vs unknown, sudden turn agression.
 

I would only continue here with a very specific read. The jack is not a very good card for you at all. You're basically ahead of Acxc and like 8c9c, but AJ, 66, 77 seem much more likely.