In an effort to help resolve his debt problems, Sumner Redstone has sold his controlling stake in videogame company Midway Games Inc. to a private investor. Mr. Redstone's holding company, National Amusements Inc., is expected to announce Monday that it sold its 87% stake in Midway to investor Mark Thomas, a move that represents a significant loss on the media mogul's investment but secures a hefty tax benefit as he negotiates other asset sales. National Amusements is in negotiations with its banks to restructure its $1.6 billion debt pile after breaching one of its debt covenants. As part of the restructuring, the Redstone family has been discussing selling some of its assets, including some movie theaters and a holding in slot-machine company WMS Industries. The sale of the Midway stake, agreed upon Friday, wasn't conducted as part of a deal with the banks but is likely to ease the pressure on the Redstone family and potentially contribute to a final deal.