1997 Buick Riviera, 42,000 miles. Always maintained very well. Has been shifting very hard at time. Doesn't always act up. Will shift very hard going into overdrive. Sometimes so hard you would think it would be it's last and going to jerk you our of your seat. I have heard it could be a solenoid in the tranny. What would be the name of it and any other suggestions would be appreciated.