The S148 servos are not too weak for a .40-.50 size airplane unless you have extraordinarily-large control surfaces or the airplane will be flying extremely fast.  That size servo used to be routinely-used in .60-size Pattern aircraft.  Your difficulty stems from the fact that your servos are old and are most likely no longer working at their intended power and speeds.  If you replace them with equal or slightly-stronger servos, you'll do very nicely, indeed.

Here's where you can look over all the current-production Futaba servos:

http://www.futaba-rc.com/servos/index.html