New 940 Owner - Diagnosing Rough Running

hello. i just bought a 1995 940 for my niece to learn to drive with.
the car idles beautifully. but when i give it throttle it sputters like crazy. if i give it almost to full throttle it runs prefect. it's just that mid throttle that runs horrible.
the guy who sold it to me had recently done the following:
timing belt
new plugs, wires, cap and rotor
fuel lines and fuel filter
compression test at 170 psi all across
i did find a couple vacuum leaks and plugged them up. he also put an aftermarket maf sensor. i put the original back on and it runs horribly. i bought a second hand maf and it also runs horrible. so far it runs best with the aftermarket.
so my question is what should i check, try, test etc to see if i can get this sputter to go away and get the car to run smoothly?
this is my first volvo and it looks like a really simple motor so it can't be all too difficult to find the problem
thanks in advance for any suggestions