Signal intensity drop over time

We are using a Quattro Micro attached with a Acquity UPLC system. We are using this for the analysis of acrylamide. The mobile phase is 0.1% acetic acid in water and 0.1% acetic acid in methanol. We replaced the capillary, clean outside of the cone and the signal was good. However, the signal keeps dropping over time (within an hour time frame). No matrix has been introduced yet. So matrix effect can be ruled out. We also tried vials from three different vendors and still the same effect. Likely interaction with glass vial is not the reason (we think). We tried polarity switching to eliminate any charging that may be occurring but no improvement. Any ideas on what is going on and how we can fix this issue?