It sounds like more than one thing, but I also would go at the fuel pressure. Replace the filter any time the fuel pressure is suspect, or the age of it is unknown(new purchase). If the pressure drops below 55psi ever, that's a big telltale sign of a bad pump. My 99 had to get that low before it finally threw a P0171 code, I kept seeing pressure in the upper 50's to a high of 60. I had a random lean(lack of power at higher speeds) issue, over about a year. When the code finally hit, then the pressure check showed 52psi or so, and I changed the pump(it had a small cut in the rubber hose at the FPR).
There is likely a wiring issue somewhere that's producing the faults for CPS, TPS, etc, the falling rpm's, tach etc. I'd keep looking hard at the wiring, all of the connectors, hunt for a chewed or crushed section of wire etc.
How do the spark plugs look, how big are the gaps? Old plugs can finally kill coil packs and lose power enough to feel it.