Welcome to the world of RWD Volvos. I agree 100% with Summerduckman, and will add the possibility of a fuel hose that may be seperating inside and collapsing on itself. I would think the line from the in-tank pump to the main pump most likely, but I can think of a situation where any of the hoses could cause this problem. I have seen this problem with an oil pick-up line in another car and have heard of this type of failure on brake lines.

Did you see any evidence of rust in the gas when you changed the fuel filter. My daughter had an '81 that sat for a while and the whole fuel system was infested with rust - we went through two main fuel pumps figuring this one out. Your '91 should be less susceptible, because the tank is not metal. However the main pump sits low in the system where moisture may tend to collect and if the roller impellers got rusty, it could cause such a problem.

Your symptoms do sound like fuel starvation, however extremely rich running can behave similarly. Check the spark plugs for carbon fouling.

Check all vacuum hoses and the air intake hose, especially the corrugated section. Air leaks can cause a lot of problems.

I don't recall anyone suggesting that you check the Fuel pressure regulator. This is a small canister that mounts on the front of the fuel rail. It is about 2" in diamater and about the same length. Remove the vacuum hose at the front and if it is wet with gas, it needs to be replaced.

You may want to measure the current to the main fuel pump when it is running right and when it is acting up. If much different, this could indicate a restriction.

I don't think it will help with your current problem, but you should clean or replace the flame trap -- see the 700/900 FAQ. The engines are the same. Lots of good info there for you to start reading. If you are going to be serious about maintaining your car I recommend the Bentley manual, It is costly but worth every penny. By the way, myself and many others recommend fcpgroton.com as a great source of parts. You may also want to find a self-serve junk yard in your area - great source of parts.

Good Luck. You will be very happy with this car when you get the bugs worked out. Hang out here and you will get the greatest help available anywhere. I have never seen another auto forum that even comes close.

Dan