La Sportiva's TRKs ($215; www.sportiva.com) are a moderately heavy boot. So perhaps more break-in will help. As the boots' leather conforms more closely to your feet, they should slide around less. Also, it's useful to tighten the laces when starting a descent. That will help control your foot and prevent it from sliding forward. You might also want to replace the boot's insole with a better-quality after-market product, such as a pair of Superfeet ($30; www.rei.com). This is an excellent insole that improves foot stability and offers more support and cushioning than a standard insole. They alone may solve the problem. Superfeet are available as an off-the-shelf item, but a dealer can also order a custom model that may be a better solution given your high arches.
But, short of that, try the lace trick and replace the insole. I think you can get the boots to work OK with some minor fine-tuning.
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