If the computer is dry and clean, the next suspect is the fuel. Water in gasoline is common. Here in California, a gasoline distributor got busted a few years ago for blending waste industrial solvents in with the fuel. That was quite a scam. They were getting paid to dispose of the hazardous waste, they were getting paid for the waste at fuel prices, and they did not have to pay taxes on the gallons that were waste because those gallons never officially existed in the fuel supply. Chlorinated solvents do not burn well and corrode fuel and exhaust system components.
You can try a bottle of HEET to put any water that may be in the fuel into emulsion so it does not come through in slugs.