Lost, possibly multiple issues
I have a marquis that's has baffled 4 shops. Sometimes it
runs fine. Usually when I take it to a mechanic. However
when it acts up sometimes it stalls everytime I hit the
brake. But can keep it running if I just tap brake and then
accelerates fine. Other times it's the exact opposite. There's
no issue with my foot on brake, but when I give it gas it
tries to stall. But again if I tap it and get my foot off the
pedal fast enough it stays running. I've cleaned mass a/f/s
and replaced cracked airbox with c/a intake
Get up under the dash and listen for a slight hiss when you start to push the brake pedal with the engine running, if you do the the brake booster is bad. Another thing is a bad spot in the TPS, a shop can test it using a DVOM and watching for a drop out during the swing, bad ones will have it usually just as the gas pedal is pressed. The shop needs to be experienced in old Ford OBD 1 engine control systems.
Disconnected the vaccum off booster and plugged it and still did the same thing when hitting the break. So it shouldn't still die if bypassing the booster if I'm correct.
Ok so you ruled out the most common problem, so next is the TPS check. I still recommend a shop that is Ford OBD 1 knowledgeable. I know a lot of shops just don't like working on them cause you need special equipment like a breakout box so you will need to look around.
Why not just replace the TPS?
Should test it first. You just don’t start throwing parts at it hoping you inadvertently fix it.
If it walks like a duck, squacks like a duck, it's probably a duck.