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The 'freewheeling' if intermittent, has nothing whatever to do with belt tension.
Belt tension normally should be just tight enough for a belt to not slip, no tighter.
Loosen the belt again, to that point. Or it will suffer wear and tear, wear out the bearings of the machine, and prove very expensive.
As for the intermittent slippage... the resistance motor (and/or controller board for it) may be ... well, intermitting. Occasionally subject to complete disconnection, loss of control, loss of power, or similar problem.
The machie in question will require repairs, for sure.