My 89 F350 with a 351w is chugging/jumping at low rpms. It seems to run fine at higher rpms. It has been running rich also. The MAP sensor reads 2.5 volts with 20 inch lbs. hg. Is that too far out of range? It was running badly before at higher rpms. I changed the ignition module and o2 sensor. Now it is, like I said, chugging and also hesitating at low rpms. I am at my wit's end. Can someone give me some direction?