Tide150

My '87 F150 302 has just started either shifting into OD or locking the torque converter at a very low speed, causing it to chug and bog. To be honest I cant tell which its doing. At full throttle its fine, but any other time, at 35mph its in overdrive. It downshifts fine and the TC seems to unlock when its supposed to. Any ideas on what to check first?

thanks