There's a letter (SB?) from Pratt about runaway JT8Ds as found on -200s. There is a shaft from the N2 section supplying feedback to the FCU. If the shaft fails, the FCU goes to a preset setting which IIRC on twin engine planes is 100%. There had been 30 some cases of the failure mode in the mid 90s when the letter came out.
As to the OP's question, I'd land the airplane.
