that does almost sound EGR related, although the complete lack of power is baffling. EGRs do tend to lead to consumption issues and lacking of power at the bottom end, suprised it would make it stall though.
what an EGR is: stands for Exhaust Gas Recirculation valve. its a bit of an emissions device that passes exhaust gas back through the manifold (from the exhaust, obviously) to go through the combustion cycle again, the end result supposedly that there is less harmfull emissions from the car.
The EGR unit is made up of two seperate parts that bolt together, the valve body and the actuator and it sits under the top right hand corner of the engine cover as you stand looking at it. The actuator is black cylinderical thing with the plug right on the end and the valve body is a rectangular metal block. you cant miss it, if you follow the black inlet pipe from the wing towards the engine, where the pipe is clipped to the engine, the bit thats bolted onto the engine intake between engine and the bulkhead is the EGR.
Problem with them is the soot that passes through blocks them up, so the valve body itself gets stuck, the actuators dont usually cause any issues as its an electrical reaction, only 1 moving part.
You can clean the EGR valve for yourself if you've got basic mechanical skills, I clean mine of from time to time.
if you take off the engine cover, you can see the EGR valve. Remove the two large bolts that go through the top of it, holding on the metal pipe that feeds exhaust in to it. be aware there is a metal gasget on this that you want to keep.
Then you can remove the 4 bolts that run horizontally through it from back to front, two top, two bottom. All that is left after these is the electrical connection on the actuator. simples.
to disassemble further, undo the 4 torx screws that hold the actuator to the valve. the actuator rod should be sticking out the end of the actuator and you should be able to push this freely in and it will pop back out no trouble, if it does, thats fine.
The valve itself, you want to be able to push right open so it hits the valve body. it will make sense when you do it, there is a shaft that runs through the valve body which is spring loaded and has the valve plates on it, the end of the shaft should be able to be moved right until it hits the valve body. it should be light enough and easy to move its full deflection, albeit with the resistance of the spring. If its stucking, won't go full deflection or won't return instantly once released, it needs cleaning.
To clean mine, I start off my removing the light stuff with a detailing brush. Think toothbrush that has metal brushes - available in halfords. Then i just the metal gasget mentioned earlier, make up a carboard gasget or two and run some bolts through to blank off that side of the valve. Then I fill it with carb cleaner type spray. Make sure you open the valve as you fill so it fills both sides.
Then i leave it to soak for a while. The gasgets I made up were only old cereal box and not up to the job and leaked a bit but still works as I wasnt bothered about making a mess. Once soaked spend a good 10 mins or so with it just pressing and releasing the valve until it feels silky smooth. then dry out and reassemble and you should be good.
admitedly it will only last so long before its completely warn out but my jdt 16v 150bhp 156 is still on its original at 86k and its going fine!
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso