I'm bowling twice a week on sport conditions. If I happen to have the time, I'm practicing once a week on a house shot.
yep. I shot 523 the other day on a house shot in a tourney. Went out a couple of nights before and practiced, shot around 625.
Now this is a house I don't normally bowl in, but geesh!!
Awful.

Now - on the reverse side of that - I bowled a scratch tourney a couple of weeks ago on a house shot - going into the last game of the finals, I was leading by 127 ahead of the next closest bowler. (and that was for the first 2 games...)
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All things are difficult
before they are easy.
---- Thomas Fuller

Not too too long ago, someone posted in these forums a great explanation of why better bowlers who are more accustomed to a tough shot, sometimes don't score that well on a soft "house" shot.  Like you, I bowl a PBA league on Tuesday night, then a "typical" league complete with house shot on Fridays.

Since your scores start out well and then decline on the house shot, I can think of two possible problems:

1) You're getting mentally lazy, even if it is subconscious.  You know on those tougher PBA patterns that every shot counts.  As you bowl more games on the house shot, seeing errant shots not penalized, do you continue to bear down and focus on execution as much as you would on the PBA patterns?  The mood during recreational leagues is often not the same as that of a more competitive league.  Make sure your mental game isn't slipping.

2) Transition and adjustment.  On the PBA patterns, we often need to make adjustments that would not work on a "house" shot (and vice-versa).  Also, the PBA patterns are often "broken down" by the better bowlers bowling on them where the "house" shot is often broken down by people using plastic and spraying shots from the middle of the lane out.  Again, each case calls for a different type of adjustment.  Maybe try to keep your adjustments on the house shot simple.
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Kill the back row (or maybe this should read "make your spares, dummy")

Gee, and I thought it was just me.    

I've always had the problem of struggling on the THS conditions when everyone else is shooting lights out and then bowling my average when the lane condition is more difficult and everyone else's scores are way down.  When I bowled on the 'sport' league we had last year, I averaged the same as I did on the THS and everyone else was lower than their THS average.

I definately need to learn how to play a THS.  
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I just want 2C was'zzub.
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I am the SGT Schultz of bowling.
"I know nothing!"

janderson, I'm sure your first suggestion is right on the money.  Unfortunately, I know it is all mental, and am having major difficulties doing anything about it.  I try to keep my head in there, but just can't seem to do it.

It's good to hear that I'm not alone on this.  I can easily go out and average 190-210 on PBA patterns now, but seriously struggle to even shoot 200 on most THS patterns.  Once in a while, however, I'll start out good enough on the THS to push me over the edge before losing concentration during the end of the tournament

I guess I just need to practice concentration more.  Perhaps I am concentrating too much on results rather than where exactly my ball is going.  Hmmmmm
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Brian
MichiganBowling.com
http://www.MichiganBowling.com

Famous Last Words of a Pot Bowler--"Ok, but this is my last game!"

Im bowling better on the house shot now because its so much eaiser compared to my sport shot league.
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The Bowlers Alley
you can roll the dice until they call your bluff
but you can't win until you're not afraid to lose