W116 1980 350 SE V8 Cold running/WUR problem

Good Morning Folks.I have a strange 1980 W116 350 SE V8 problem. when car is cold it does not accelerate well, cuts out and then struggles to start. When it does drive it does not pull and splutters all the time. Once its fully warm it then drives ok with and occasionally will not respond to the accelerator. This points to warm up regulator. I did the following over a period of time before getting to WUR.1. New coil and new plugs.2. Checked and changed rotor.3. Changed fuel filter and fuel pump.4. Warm up regulator cleaned reset.5. Replaced Warm up regulator with used one cleaned and reset.6. Removed tank cut open cleaned and welded back together.7. Fuel accumulator is relatively replaced with new part a few years ago.After all this problem persists and only goes away after putting in fuel line cleaner fluid. The car functions fine for a few days and few weeks maybe.8. This weekend found two used WUR with no change when cold and again after putting in a bottle of fuel line cleaner car functioned well, accelerated etc.Help....dont know where to go from here.William DavySouth Africa