, The cost of getting a spark module repaired will be more than the unit is worth. one or two of the internal transistors have probably died or gone high resistance, hence the module cant deliver the voltage quickly enough to the coil at higher rpms, so rough running at high rpm or when the module has got warmer.
Its possible that water from washing has got inside the module, offering a least resistive current path over a few of the transistors once they have got hot. try opening up the module and putting it in the oven for 10 minutes at 30 degrees to eliminate any water near the transistor joints. also check for dry joints on the pcb, Failing that, identyfing and replacing the faulty transistor/ transistors. perhaps a small local electronic shop would do it cheaply for you to make it worthwhile, or do it yourself for the cost of the trannies