Sounds to me like those pots shouldn't have been rated anywhere near 3 Watts.
If the pot was wide open (max resistance) and the voltage across it was ~6V, then the power dissipation would have been:
P = V^2 / R = 6^2 / 28 = ~1.3 Watts
Nowhere near the 3 Watt spec. I'd just chalk that up to being sold junk by Radio Shack. Not too surprising.
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