...when the engine shuts off with ignition key on, the reading is 100% accurate.
There's no such thing as 100% accurate. Repeatable maybe, but not accurate.
Back to the original question - it's normal to see noise on electrical lines. The noise is caused by spark plugs, distributors (both electronic and otherwise), electrical fans, pumps, radios, etc. Only worry if the TPS % readings drift by more than, say, 2% with the engine on. It's no wonder the TPS is stable with the engine off...