I'd guess you're swinging too hard with the 5-iron, leading to inconsistent contact and loss of distance. Maybe swing a little easier and concentrate on clean contact. The shaft length and loft will provide all the power you need.
Trying to swing too hard sometimes lead to throwing the club from the top also; swinging easier will help delay the release, increasing lag and thus power.
I wish I had your problem though, haven't played in months. Stupid winter.