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There are a few things that can cause this: Stuck servo for the front band, front band burnt out or maladjusted, stuck accumulator piston in the valve body, low trans fluid, broken or weak relief spring in the pump.
Check your fluid to see if it shows signs of being low or burnt. If this has happened after a recent fluid change, you're looking at a rebuild. If the fluid is OK, try adjusting the front band (you should do both while you're at it, not just the front one). If the problem isn't fixed, you can try looking for a stuck piston in the valve body, but at this point you'd be looking at a rebuild anyway.