I have a 95 I'm having the same trouble with. I've searched numerous forums etc... on how to solve the problem with no specific answer. Hesitation starts just off of idle and ends around 2000 RPM. I have replaced many many things and have maybe narrowed it down to the EGR Valve (not the valve solenoid which I have already replaced). If I disconnect the wire going to the EGR solenoid, to stop the function of the EGR valve, it runs pretty good. But that doesn't mean it's not another problem the ECM is compensating for. Search message boards and you will see this is a common problem among 305 owners. So far no one has found the specific source.
Most Common are:
EGR Valve, Opti, Coolant temp sensor, IAC sensor, or injectors.
It seems as though many differant problems cause the same symptoms.
Good luck, and if you find a solution before I do, please let me know. -Dilve