assuming your main panel and associated incoming cabling are appropriately sized for the loads you have then you should not experience more than a 10% voltage swing under "normal" circumstances for your local utility.
if you have a multimeter you should record incoming line voltage at various times. if you are experiencing voltage swings like you described (~40%) there is no utility in the country that isn't legally bound to correct the problem (assuming you don't have the problem).
if these conditions existed since installation then a severely overloaded transformer may be the problem. if it is a problem that has developed over time then i would suspect something is failing. it is not uncommon for overhead lines that are run through trees to eventually have a limb contact the line and through wind action eventually rub away the insulation in one spot and cause voltage fluctuations. the utility can verify the integrity of the incoming lines with a simple megger test.
call the utility and see if you get an appropriate response. if not then definately put it all in writing and send a certified letter.
if you have a multimeter you should record incoming line voltage at various times. if you are experiencing voltage swings like you described (~40%) there is no utility in the country that isn't legally bound to correct the problem (assuming you don't have the problem).
if these conditions existed since installation then a severely overloaded transformer may be the problem. if it is a problem that has developed over time then i would suspect something is failing. it is not uncommon for overhead lines that are run through trees to eventually have a limb contact the line and through wind action eventually rub away the insulation in one spot and cause voltage fluctuations. the utility can verify the integrity of the incoming lines with a simple megger test.
call the utility and see if you get an appropriate response. if not then definately put it all in writing and send a certified letter.