Heres my problem recently did an alternator on my 03 MPV over 100k miles.
What a bear also replace ball joint. After I got it all back together I took it for new front tires. It had been sitting in my garage for a year before I got around to repairing it.
As I was driving it more home after tires noticed it running rough I filled up tank as it was low on gas. Also put fuel injection cleaner in full tank & some gas line anti freeze in case of moisture in tank while sitting.
As I was returning home it got louder & I barely made it up my hill no power.
When i pulled in driveway it was gettting darker I could see the flex pipe was red hot what was left of it also could see flames shooting out like raw gas.
No check engine light is on brought to shop to put on scanner & they got no codes either.
Sound like you have a plugged exhaust you said flex pipe . Is cat converter still on it? DO a simple test loosen exhaust before cat if their. If not loosen at first flange after manifold. Just loosen dont remove. Loosen enough to have a gap between pipes the test drive