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What, exactly, is the problem? Most miter saws, even the $150 contractors' versions from home improvement stores, will cut very accurately if adjusted properly. Before you buy a new saw, have you adjusted the one you already have? If the saw is old, worn and loose, then it would be time for a new one. However, my nearly-decade-old Milwaukee, Makita, and Delta saws still cut accurately.
Also, are you sure the moulding pieces are not moving during the cutting process? Any movement of the moulding during cutting (or joining) would result in poor miters. Saw fences, such as
Clearmount,
FrameWorx, or the now-defunct Phaedra (excellent, and still available on the secondary market) provide accurate measuring and hold-down devices suitable for professional framing. They are well worth their cost of a few hundred dollars.
Or, if you plan to build dozens of frames per week, then a good used double-miter saw might be your best investment.