The Air Force, already faced with guidance control problems on MX nuclear missiles, said yesterday it is withholding $1 million a month in payments to Morton Thiokol Inc. because of "poor workmanship" on the missile's rocket motors.

Air Force officials said sloppy management and workmanship caused delivery delays in the first-stage rocket motors produced by Morton Thiokol's Brigham City, Utah, plant.

"At this point, no impact to the overall Peacekeeper {MX} deployment schedule is anticipated," an Air Force statement said. "However, internal Morton Thiokol production schedules have been missed, which, if left unchecked, could impact on future delivery schedules."

"Yes, there was a problem," said Thomas S. Russell, vice president of corporate development and strategic planning for Morton Thiokol. "I think we have solved it . . . . The last three deliveries have been on time. We are confident there is no defective delivery of any stage."

Air Force officials said the decision to withhold the $1 million -- 10 percent of the monthly payments to Morton Thiokol -- followed efforts since December 1985 to improve the contractor's performance.

The Air Force withheld $1 million from the company in February for cost and control system deficiencies. A June audit by the Ballistic Missile Office "found continuing workmanship problems." The Air Force then decided to withhold $1 million a month until it is satisfied the problems have been corrected.

Morton Thiokol's Wasatch Operations Strategic Division in Utah also produced the faulty solid rocket motor blamed for the space shuttle Challenger explosion in 1986.

Congressional hearings revealed that seven of the 21 MX missiles cannot be fired for lack of guidance systems and others cannot hit their targets. The guidance system contractor, Northrop Corp., has disputed some of the allegations.