Wavy rolling baseline in diluent injections....

I am using a Waters Alliance HPLC with a Waters Spherisorb ODS-1, 5 micron, 4.6 x 150mm column.  My mobile phases are: Mobile Phase A (17:83 ACN:water with 0.045 M Sodium Phosphate, 0.006 M SDS, and 0.002 M Zinc Sulfate), and Mobile Phase B (26:74 v/v ACN:water with 0.094 M Sodium Phosphate, 0.006 M SDS, and 0.014 M Zinc Sulfate). I have a gradient profile of 0% - 90% MPB in 30 min with a flow rate of 1.5mL/min from 0-25 minutes and then a flow rate of 3.0mL/min from 26 - 30 min.  

I have only recently been experiencing a wavy rolling baseline in my diluent (100% MPA) injections from 2.0-26.0min.  I simply cannot get rid of it.  My method parameters and gradients are locked and not open for alteration. 

1. I am wondering why all of the sudden I am seeing this effect, and what is giving rise to it?
2. Is there a means of getting rid of this rolling baseline without significantly altering the existing method and having to re-validate from the beginning?
3. Because I have a varying flow rate in the method, Empower will not allow me to alter the stroke volume from 100 microliters.
4. In the flow tab of the method, I have the curve settings set at 6.  I have never had to change this default setting.  Therefore, I have no understanding what lowering the value away from the linear slope setting of 6 towards 1 or raising the linear slope setting towards 11 would potentially do to my baseline.  Does anyone know if this might resolve my issue?  If so, what would be the desireable setting?
5. Also in the flow tab of the method, I could potentially change the accelerate to 10.0mL/min in: 2.0 min default setting.  Again, I have never had to change this default setting.  Therefore, I have no understanding what lowering or raising the value away from 2.0 min would do to my baseline.  Does anyone know if this might resolve my issue?  If so, what would be the desireable setting?