Sounds like your labs are running the film through faster and rasing the temp. to compensate to save time and not replenishing their chemicals on time to cut costs. I have even been to Kodak Q labs {the best of the best suposedly} that do this. Just keep testing labs to find the best one. Here is a sugestion expose 5-10 sheets of film at the same exposure of the same seen and send one each to 5-10 labs. You will be suprised at the difference.