1995 Nissan Pathfinder Driving

My problem is that it shifts fine but it seems like it won't shift into overdrive. It runs at 3000 rpm before it shifts and is very slow to get to about 55 mph. Once it's there it won't go hardly any faster. When you let off the gas it immediately tries to slow down as if they're is a parachute behind it. But the thing is once in a blue moon something will change and it will drive perfect. But without change or warning it will just drop back into being sluggish and not shifting out at 1500 rpm neighborhood. Please help!