Suzuki XL7 Oil Pressure Light Flashing On & Rough Acceleration when out of Overdrive

My mother's 2002 Suzuki XL7 Grand Vitara 4WD recently started having a problem with the oil pressure light flashing on while driving. This usually only occurs when she's in drive; if she switches over to overdrive, the light tends to go out and stay out. The vehicle seems to also accelerate much rougher in drive than it once did. Again, acceleration seems normal in overdrive. We have checked her oil levels and everything seems fine. She recently had a local mechanic replace her fan motor clutch and spark plugs. She had been having rough idling problems and problems with her air conditioning. Since this oil light problem happened shortly after that, I was just wondering if something might have gone wrong during repairs. The mechanic is a trusted friend of the family, but I know no one is perfect. I know the oil pressure light coming on can mean many different things, from faulty oil pump to sensors, but I figured I would ask on here to try to narrow it down by describing the situation. Thanks in advance for any help!