2003 caravan 3.8 v6 with auto & a/c

I'm having problems with the drivability of my car. The engine misses when I gently accelerate between 40 to 50 mph. If I give more gas and it shifts into a lower gear it seems to smooth out. Also another problem (may be related) is that the tach will jump for no abvious reason when I'm coasting down hill (tach will jump up and downseverl hundred rpm on scale, without any noticeable change in speed on speedometer or engine speed)