To check if it's the stat start the engine from stone cold. Constantly feel the top and bottom rad hoses. If they warm up together then the stat is open. If the top one gets very hot first then at about 90C the bottom one starts warming up then the stat is ok. However the temp dropping to 70 when on the move but rising to 90 when stationary is classic stat symptoms. You're quite clumsy so I'd order a new stat housing. With age and the heat cycle they go very brittle and very easily crack or break off into bits. All of the coolant will drain too, as the stat is at the lowest point in the cooling system.