Is there other code trying to talk i2c to the pcf8591 or any other i2c device? The timer2 interrupt could be jumping in the middle of such a transaction thus causing some confusion.

Generally if I design a product, we're talking $1000's of dollars in design time and pcb prototyping. If I'm only going to make 120 devices a year - a few dollars per micro to save my time is a fair tradeoff. The economics may be different in your part of the world but generally engineers are paid better than McDonalds staff.