I have an '88 760Ti with 203K miles. This past January, the original main fuel pump died. I had it replaced with a Volvo OEM, purchased and installed at the dealer, new main pump (because I was on a trip, far from home, and it was snowing horizontally).

I also replaced the fuel pressure regulator in March. I now have the same symptoms, it'll run when cold, but eventually (between 40 to 60 min) will cough, lose power, and stall. After 20 min or so, I've been able to restart it and get to where I want to be.

There's no smell of gasoline at the regulator. The pump powers up when I turn the key to "on," at least when it's cold. I realize I've been too frazzled to think about listening for it when it has stalled.

My question is, how do I tell if the pump or regulator might be at fault? Or maybe something else. Does the 700 series suffer from the same plastic line failure above the intank fuel prepump as the 200 series?

Advice or suggestions, please? Thanks.

If you have fuel at the regulator the fuel pump isn't at fault.

Sounds like you have the standard/pattern failure Bosch fuel pump and RF relay failures.

Remove and resolder the fuel pump relay and RF suppression relay.

OTOH, it might be something as simple as dirty throttle body, clogged air filter, bad plugs, bad sparkplug wires, etc. etc.

We recently had problems starting the 87 760T w/250K miles...I had fuel, spark, compression, etc. Soldered fuel pump relay, worked for a couple weeks. Car wouldn't start, towed, etc. Dealer insisted plugs were bad (mebbe so but I pulled the plugs and they looked clean, gapped, and were less than 30K miles!) and put new ones in and car started so go figure.

In the past 5 years I have had more problems with plugs.
Never in 18+ years have I had a bad plug out of the box. These last bunch of years, I have had about 4. One on my boat engine (NGK) two one the Motorcycle(NGK) and just two weeks ago I had a bad one in my Pickup truck (Autolite).

Since they are new, it's the last thing you look for when the vehicle starts acting up.
I don't know why they dropped the ball on QUALITY but obviosly they did.

I use to have the spark plug problem on a particular car. Put in fresh plugs and the car would run strong for about 2000-3000 miles then very gradually it would get a miss or ping, then a little more till it was really missing and had no power.

The plugs were very clean and showed no signs of damage. The plugs had just a slight grey color on the ceramic, but what was happening was the coating was grounding out the electrodes so no spark. I never could stop this so got into the habit of replacing plugs when needed.

Of course this was a high performance, high compression motor (289 cubic inch, 335 net horsepower), but I would almost bet it was the gasoline sold during that time period. (96 to 105 octane with lead)