Slight juddering engine.  Try 500ml of RESTORE.  It will smooth out the compression for you across all the cylinders.  In a petrol engine there could also be arcing (high voltage tracing) inside the distributor cap.  Have a mechanic remove the cap and look for telltale white lines on the inside black plastic , or wait till dark and lift up the bonnet and look for blue sparks from any of the high tension HT cables, or you can also try taking a water spray bottle (like you might use to wet down a plant's leaves with) and with the engine running, lightly mist all over the HT high voltage leads.  If you get an engine hesitation and/or a blue flash, there is the problem.  There are HT spray sealers available on the market that work quite well.  If there is nothing to be seen, try a fuel injector cleaner liquid into the fuel tank.  Finally, make sure all electrical connections are clean and tight e.g. battery etc.  You might also need to have the ECU (engine control unit) evaluated by a specialist.