2000 V6 Solara, automatic transmission. Just started yesterday, when I am driving at speed (25-70mph) the car will not down-shift to a passing gear, not even out of overdrive. Instead, the car hesitates, jerks and "bogs" down until speed is obtained from the current gear. If I shift the car manually, no problems. If I come to a stop or a turn where I have little speed, the car shifts properly up the gears. Any ideas?

How long has it been since you gave your car a tune up? That might be the problem.