I own a 98 BMW 740il I drove my car yesterday.

I own a 98 BMW 740il I drove my car yesterday for 3hours straight, I parked my vehicle and after about 4hours later when I shifted into gear 'D' the car seemed heavy. I stepped on the gas peddle and it wouldn't move until I accelerated almost to the floor. I checked the tires and it wasn't a flat tire. Then the check engine came on and it gave me the obd code P1738 Pressure Control Selenoid 'C' Electrical. I left my car over night. In the morning I turned the car on and shifted into gear 'D' and the car worked normal again. On my way to the shop the car was running smooth, gears were engaging normal with no grining or jerking motion. On the freeway driven the car approximately 30 minutes about 8 miles the car went into FailSafe mode again. I need help to see what can cause this to occur. The mechanic said I need a new transmission but when I drove the vehicle the gears were going in smoothly with no problem. Thank you.