serena engine problems

hello and thanks for taking the time to read
my problem is with a 1993 serena 2.0 slx
at christmas engine started ok but tickover was poor and no pulling power in the engine
cleaned the distributer cap and rotor arm ,this sorted it
the problem reapeared this month so fitted a new cap and rotor arm
still the same
then fitted new leads
then new coil
still no fix
engine revs up but seems to be misfiring
it does run but totally lacking in power
any advice welcome