Sounds like a fuel probelm. Have you "read" the plugs right after a hard run? If you havent, you should. Give the car a hard run, then pull a couple of plugs out. If they look like brand new, or white, you need more fuel. Ive even seen the same thing happen when the floats are too low, especially the rear one. Keep us updated.... Also springs might do the same thing, but you say youve checked them.
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