oly 14-150 prone to softness and flare?

I've had the m.zuiko digital 14-150mm for some time now. It came in a used kit I got from ebay. The front and rear elements both appear fine, but I struggle to get sharp images when the lighting is flat, and flare is pretty obvious (but fun) when shooting into the sun. Dunno if this is due to the lens design or the cheap 7dayshop UV filter I use as a protector. Any idea?