I've had the exact same problem. For a year, we tried different brands of detergents, different amounts of detergents, different brands of rinse agents, adding vinegar in varying amounts, etc. and never got it licked.

Then, I read a NY Times article about the general dissatisfaction with the new phosphate-free detergents. It never occurred to me that switching detergents wasn't going to work because all detergents were going to be equally bad. Even though my state doesn't ban phosphates in dishwasher detergent, it seemed like all the national brands were either selling new formulas in all states (like Cascade), or the places I was going were stocking from national warehouses (like Costco). We go to Mexico a few times a year, so the next time we went we just bought a big bag of the cheapest stuff that listed fosfatos in the ingredients. Problem solved. ¡Es un milagro!

With good enough water, I don't think the phosphate-free detergents are a problem, but our water is really hard, with tons of rust and who knows what else. So, my guess is the phosphate issue is the root cause of your problem, but since the timing doesn't quite line up with the switch over, it seems like there must have been some change in water quality in your case.