Semibluff or Chip Spew?

Reads:

UTG is weak passive, will pay almost any price to see his draws.

MP is a conservative thinking player. Better than I am. Has built a huge stack and has never been caught bluffing. Will not put very much of his money in the middle without thinking he's got the winner, and he's almost always right.

I'm in LP with a very loose table image. I've made at least three $100 bluffs on the turn and river, then shown my hand afterwards. I haven't been position raising very often unless I have some kind of hand.

We're playing 1-2 NL Live. UTG has $200. I have $650. MP covers us both.

UTG makes it 20. MP calls. I call with 7
[img]/images/graemlins/heart.gif[/img] 8 [img]/images/graemlins/heart.gif[/img]. Everyone else folds

FLOP: ($63) Q [img]/images/graemlins/heart.gif[/img] 7 [img]/images/graemlins/club.gif[/img] 3 [img]/images/graemlins/spade.gif[/img]

UTG checks. MP checks. I think my 7 might be good here, so I bet $15 (too little in retrospect). UTG calls. MP raises to $45. I think that represents a fairly strong queen like KQ or QJ but probably not AQ, so I call thinking I'll shut down if I don't improve on later streets. UTG calls.

TURN: ($198) A [img]/images/graemlins/heart.gif[/img]

It's checked to me. I think the A must scare MP, plus it gives me a backdoor flush draw. If the flush (or 7 or 8) hits, I want to be able to double up through MP, but he's unlikely to call a $550 push into a $200 pot. So I bet $100, looking to either fold both players or build a pot in case I hit. UTG folds. MP thinks for several seconds, then calls. Now I'm sure he has a Q, but not AQ.

RIVER: The river is a total blank, 6
[img]/images/graemlins/club.gif[/img]. MP bets $100. I contemplate a push, then fold.

Comments on all streets appreciated.