1996 Ford Ranger shifting

Everytime I start the engine and shift to Rear, the vehicle takes time before it jumps into gear, sometimes it jolts into gear, other times it's smooth. When shifting to Drive, it works well everytime. However, when driving around for 5 min or more, the rear gear seems to work better (shifts faster and smoother). Sometimes when driving about 40 -55 mph on the freeway, the O/D warning light blinks and the vehicle seems to have a hard time gaining power especially uphill. The only way I could reset this is by turning the engine off and restarting it. Could you please advice me on what this might indicate