A computer system problem that has bungled paychecks for thousands of National Grid employees for nearly two months continues to plague the company, whose president Wednesday issued an apology for the "painful and unthinkable" problem.

In a companywide note, National Grid USA president Tom King took "full responsibility" for the problem, which has disrupted the finances of thousands of workers as they put in overtime to restore power after superstorm Sandy. King said progress was being made.

"Through December 6, a significant number of our employees have had payroll adjustments," he wrote. He said fully fixing the problem was a "monumental undertaking," but said he expects to resolve "most issues" by year's end.

Separately on Wednesday, two LIPA ratepayers filed a class-action lawsuit alleging that the Long Island Power Authority and National Grid "fraudulently and improperly" billed them for daily service charges of 36 cents even though they did not have power.

The suit, filed in federal court in Central Islip on Dec. 7, seeks more than $5 million in back payments and damages, and an injunction preventing the companies from levying the charges when service is stopped.

LIPA has said it would refund $4.9 million in service charges to nearly 1 million customers. Andrew Bell, an attorney for ratepayers Harry Friedman of Woodmere and Lazar Gozenpug of Valley Stream, said the suit "would still be viable."

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LIPA spokeswoman Elizabeth Flagler declined to comment on the suit, but noted the that waiving the daily service charges for storm-impacted customers will be voted on at a trustee meeting that has been rescheduled for Monday.

Another lawsuit filed on behalf of National Grid workers takes issue with widespread payroll problems after the company implemented a new computer system as Sandy was bearing down on the region.

National Grid is setting up "payroll clinics" in operating yards and offices to help resolve this problems, and is paying late fees, surcharges and other costs of employees who haven't been able to pay their bills on time because their pay has been delayed, King's memo said.

But for members of the International Brotherhood of Electrical Workers local 1049, those actions aren't enough. IBEW business manager Don Daley said the union was considering legal options in addition to complaints filed with the state attorney general, Public Service Commission and the U.S. Department of Labor.

"I represent 2,600 workers and I would say the majority have been impacted in one way or another," Daley said. "Our members are extremely frustrated and justifiably so."