my 95 mustang v6 isn't running right.

alright, my mustang has kind of a rough idle, i had pulled the motor and put it back in, new clutch, cold air intake, flowmater dual exhaust, when you hit the gas it goes to the proper RPM, but then drops low, i adjusted the idle screw helped, but didnt fix,the check engine light is on, also when driving it, at about 60, fifth gear has no power, it is at about 1500 RPM and if you hit the gas it bulgs out, but in every other gear it has good power,motor has 105*** on it, strong motor, just has these two issues,

anyone have any thoughts?

ight, thanks, also does anyone know anything about why my rpm guage keeps working and then kicking off. same with my speeddometer, i put a different one in and still didnt solve the prblem.