i think i have some sort of fuel issue with my 94 850 (non-turbo). i'm not sure how i know to replace the relay, the pump, and or something else.

the current situation:
upon starting, the engine will die within a few seconds if i do not hold the gas peddle down and shift quickly into drive and spin-off--not good. once i'm on my way, the car will not die while driving as long as i am holding the gas peddle down. however, there will be 2-4 second periods where the car pauses it's acceleration as if it's not getting any gas. then, it will go back to normal. this used to happen only when the car was low on fuel.
when i have to stop the car, then i usually have the car in neutral while holding down the gas and the brake. at that time i can hear the engine sounds a bit rough(yesterday, i had to make an emergency 100 mile trip to see my dying grandmother. otherwise i wouldn't know all this and wouldn't have subjected my car to such abuse).
oh, and this problem just developed like 4 days ago. at first, it just died once on my way home from work at the stop light. then, the next day twice. then, yesterday, the check engine light came on and the engine will die while idling. it might be worth mentioning, even though i don't want to confuse or cloud responses, that my fuel economy has dropped by 25% or more over the last year and that fumes of gasoline are frequently present when the engine is running.

so, am i right that i seems like the fuel pump or could it be something else completely different? is it really worth the cost for someone to pull the codes? what do you think? just the pump or the relay? should i replace the whole fuel pump assembly or just the pump itself? and lastly, new or used? i'm not sure that i will keep this volvo for more than 5 years.

i realize that i could spend a lot of money trying to get someone else to figure out the problem, but i'd rather start the process of diagnosing in a systematic way. i am open to all suggestions.