Roger Clemens did it once and also had back to back seasons of 2.65 and 4.61 as well.
Glavine had a 2.47 ERA in 1998 and a 4.12 ERA in 1999 while logging 200+ innings in each season. He had a 2.96 ERA in 2002 and 4.52 ERA in 2003 while also throwing 180+ innings each year.
Andy Pettitte went from 2.88 to 4.24 in 1997 and 1998.
Clemens immediately came to mind when the OP started trying to claim that Lester's production decline was cause for concerns. Thanks for doing the research. Keep on trolling, cyclones360.....