My '94 940 (180K miles) often is hard to shift into Park. Sometimes when I think I've put it in Park, turned the car off and then come back to drive it it surprises me by being in Reverse. With my foot on the brake this hasn't caused any problems but with a new young driver in the house I'd like to get rid of this problem but am not sure what kind of troubleshooting or repair to request from my mechanic. I'd like to be able to say something more descriptive than "hard to shift into Park".

It is less of a problem when the car is parked on the level than it is when it is parked facing uphill. Actually, when the car is parked in the garage it is virtually a non-issue and it shifts into park with little or no extra attentiveness. Transmission fluid was changed about 15K miles ago.

I'd like to have some sense of how serious this is and how costly to repair because I'm on the verge of paying for a a major tune-up and don't want to be surprised by having to pay for an expensive transmission repair next year. Other than this one problem the transmission shifts smoothly and has no issues.

Any advice would be greatly appreciated.
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'94 940, 40 linear feet of vintage jazz, r'&'b, funk and soul