1990 Volvo 740 Why does car hesitate and stall?

Engine Performance problem
1990 Volvo 740 Automatic 165000 miles

Car hesitates upon acceleration from a standing stop and causes check engine light to come on and stall.

My dad checked the codes and replaced the oxygen sensor, which was the only code that came up. Coolant sensor and crank sensor also were replaced previously prior to this problem.

My dad also tested the fuel pumps and was getting a high reading without the car running. He says he needs to get some fittings and test the fuel pressure again. If the problem is not the fuel pumps, he is not sure what to do.

Also, my dad drove the car after replacing the oxygen sensor and it was still hesitating, but drove a good distance without stalling. Then today my boyfriend tried to drive it and it was stalling again.

Thanks.