Need some help with my strange engine stumble problem Options

[P.Rocket]

I've had my '74 914 2.0 for about five years. It is running on the original fuel injection.
I've always had an intermittent stumble at about 3200RPM that was intermittent enough to easily live with.
This year when I started driving it back in the spring the stumble started happening a bit more often. I could get it to stumble while sitting in the driver's seat and pushing the clutch in and out, at an idle. Oh well, kept on driving it.
Last week a drove it a lot and found that the stumbling was continuing to develop while driving.
I turned the key on one morning, heard the fuel pump run then stop (like normal), then I was moving around in the driver's seat and the fuel pump cycled again. So as I sat still in the seat, the pump wouldn't cycle, when I would move around I could hear it cycle again. With the car idling, it will stumble while moving in the driver's seat, regardless if the seat is adjusted froward or back.
Right away I thought about the seat sensors/seat belt switches or that POS interlock module under the passenger seat. So I unplugged both the drvs and pass. seat sensors and seat belt connections, the pump still cycles when moving around in the driver's seat.
I checked and cleaned the grounds from trans to chassis and added a ground strap from the trans to the engine. I also checked and cleaned the grounds that go to the chassis and mount under the relay box in the engine compartment. No change.
Ran out of time, so parked it in the shop and now onto my work week.
Thought my next plan of attack will be to pull the pass seat and check out the interlock module and by-pass it. Can someone tell me exactly what this unit does? Does it cut the signal to the coil or the fuel pump? What is it's purpose for North America only?
If anyone else has had this kinda weird problem, your input is much appreciated.