Bad Fuel? Redline SI-1 Overkill? Exhaust Leak?

[EricSilver]

I am finishing up my second bottle of Redline SI-1. (I Added a full bottle on each of two consecutive fill-ups.)

I added the second on Thursday just before a road trip from Delaware to Virginia. On Friday, I noticed the car feels bogged down -- heavy steering and not as swift acceleration. There is a low, steady, sputtering sound from the muffler (seems to be emanating from the forward-most part) that sounds like a small, baritone motorboat – most noticeable at idle and disappears at highway speed. Engine also seems to roar and fuel gauge is intermittently erratic just below the Full mark.

Prior to Thursday:
Exhaust was silent and the car behaved well. Fuel economy was normal and the gauge rock steady. Visual inspection of the muffler reveals no holes, and the sputter appears to be coming from where the pipe joins the muffler (I see some welding/brazing occurred there).

After Thursday:
* Added second Full bottle of Redline SI-1 on Thursday (Aug 13th) -- added first last weekend (Aug 8th).

* Also on Thursday car was jacked up high at the left front for about 4 hours while vainly attempting to adjust a defective NSS switch.

* On the road trip to VA I passed through one road construction zone and kicked up some pebbles at low speed. If one punctured the muffler I should be able to see the hole.

* I have experienced this bogged-down behavior once before on a trip from DE, and it was also an occasion when the car was jacked up on one corner for a long time while I replaced the NSS switch.

I passed emissions & safety inspections today with no issues. When tech lowered car from jack I noticed muffler & tailpipes bouncing, as if a hanger was missing.

Is it possible the Redline liberated some fuel system mud that is causing this behavior? (The erratic fuel gauge tells me it is cleaning something.) Or did I crack the exhaust?