4-3 Downshift Rough

[Dfishrmn]

I have recently noticed a rough downshift from 4 to 3 at lower speeds. It is fine when kicking it in to passing gear except for a lag or plateau before it starts to accelerate. At lower speeds it seems to have a hesitation then just jumps to 3rd, even at a normal downshift (coming to a stoplight, etc.) Kinda hard to explain the feeling but is not normal. This just started happening after I installed a 14" Magnaflow muffler on an otherwise stock exhaust. Bill said there was no need to re-program because of this mod so don't know what is wrong. Maybe a coincidence with the Magnaflow. I am running the shift firmness at +10 on all shifts, besides that no modifications to the custom 87 performance tune as far as shifts are concerned. Tranny had a complete flush 10K miles ago at 35,000 mi. The downshift doesn't rattle your teeth but is annoying! Also wonder why it has the "flat spot" mentioned previously when downshifting in heavier acceleration situations. I would appreciate any input!