Weak Hot Water Pressure

I have a Moen single handle tub/shower similar to this model: http://www.moen.com/shared/pdf/t2393pt.pdf
I don't have an exact model # (I can take pictures if needed). Single story house and all other hot water output is acceptable. This tub takes about 10 minutes to fill up with warm water. The cold is _okay_ but not great either. I have a large jacuzzi brand bathtub in the master that fills up an order of magnitude faster than this standard tub.
The lever does not need to be pushed in or pulled out.. just rotated for water flow. As soon as the pear shaped knob is pointed 10-15 degrees just right from straight up, the water output from the spout turns to 25% of acceptable. Pulling the pin on the spout to activate the shower head takes a while for water to come out and is also weak pressure.
Bathroom plumbing is on an interior wall with access if needed by cutting drywall from other rooms.
Is this fixable by a DIY'er? or do I need to hire a plumber?
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Moen has wonderful customer service. Try calling them. They provide free repair parts for most models.
BTW did this start recently? On one occasion we found a bit of wood in the Moen valve. It had been used for about five years and then one day slow flow. Cleaned the wood chip out and back to normal.
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