W107 350SL Fuel system question

[BBAY]

I recently picked up a 1973 Euro 350SL from an estate sale with 105k indicated miles. It had been in enclosed storage since 1992. After doing the typical (flush fuel tank, change fuel pump and fuel filter, replace plugs, clean injection trigger points, set timing, it started and seems to run well. On the first test run, it ran well at speed for about 5-6 miles, then began to have an engine cut out every two seconds or so. The longer the run the worse it became until finally only managing 20 MPH. The cut out frequency corresponded with the change in tone of the fuel pump. (spin-rattle-spin-rattle-spin-rattle=run-cutout-run-cutout-run-cutout). I checked the fuel tank and saw some contamination so reflushed and replaced the new pump with another and installed a new fuel filter. Same story. Runs fine for 2-3 miles then starts cutting out. A feathered throttle seems to allow some normal running up to 2000 rpm. Great the throttle open the more pronounced the cutting out. Let in rest for 5-10 minutes and the 'run well for several miles then cut out' cycle repeats. I'm think possibly a fuel pressure regulator problem but thought I would put it before the board before digging deeper. Ideas?
Thanks
BB