add oil to the highest level you can to the tank.
check the duty cycle of the building and assess wether it has changed.
add more time to door dwell for a longer time between travel of the lift.
if all else fails and/or you are unable to change anything the only real solution is an oil cooler. ive installed these coolers to hydro's on my round where lifts were constantly going out of service to zero call backs. they arent cheap but you need to sell the idea to the customer what would you prefer? o-o-o lifts or lifts that work.
http://www.dicksondata.com/products/SP125
Hydros put out about 20,000 BTUs an hour, (very roughly, and of course depends on a myriad of factors). The a/c system needs to be able to extract that amount of heat out of the machine room, times how many elevators are in there. Many architects overlook this, to the customer's detriment.
Like greenpants said, reduce leveling times. That helped me once.
Maybe the oil is losing is lubricity? There are friction modifiers you can add, short of changing the oil. Your local supplier should be able to test a sample of what's in your tanks for you.