bottom set vs slowplaying PFR

[Paul2432]

This hand occurred in a WSOP Step 3 ($200+20).

Party Poker No-Limit Hold'em Tourney, Big Blind is t15 (10 handed) converter

SB (t970)
BB (t535)
UTG (t1000)
Hero (t930)
UTG+2 (t975)
MP1 (t940)
MP2 (t925)
MP3 (t730)
CO (t2055)
Button (t940)

Preflop: Hero is UTG+1 with 6[img]/images/graemlins/club.gif[/img], 6[img]/images/graemlins/spade.gif[/img].
UTG calls t15, Hero calls t15, <font color="#666666">1 fold</font>, <font color="#CC3333">MP1 raises to t99</font>, <font color="#666666">3 folds</font>, Button calls t99, <font color="#666666">2 folds</font>, UTG calls t84, Hero calls t84.

Flop: (t421) 6[img]/images/graemlins/diamond.gif[/img], K[img]/images/graemlins/heart.gif[/img], J[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(4 players)</font>
UTG checks, Hero checks, MP1 checks, Button checks.

Turn: (t421) Q[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(4 players)</font>
UTG checks, <font color="#CC3333">Hero bets t300</font>, MP1 calls t300, Button calls t300, UTG folds.

River: (t1321) 4[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(3 players)</font>
Hero checks, <font color="#CC3333">MP1 bets t541 (All-In)</font>, Button folds, Hero calls t531 (All-In).

Final Pot: t2393

On the river here, MP1s play looked very suscpiciously like a flopped set that decided to slow play. Does anyone fold here? Comments on earlier streets welcome as well. (Preflop, I told myself I would call if two others called after the PFR. On the flop my plan was to check raise all-in.)

Paul