Shifting issues

I have a 1999 STS with 80k. Recently when I shift into drive it feels like it is dropping into gear way too hard. There is a "thud" noise and the front end of the car will bounce slightly. Shifting into any other gear is smooth as butter. It happens probably 90% of the time. The rest of the time it goes into gear smoothly. Also when I pull away from a stop (already in gear) there is a slight delay with a very similar "thud". Occasionally at cruising speed when I coast from say 55 to 45 and get back on the gas I have the same delay and "thud". Once I'm going I can put the hammer down with no problems whatsoever.
Please help!!!!

May be time to check engine and trans mounts.
Or anytings else that may be lose.
MT

Yeah, the motor mounts are a fairly common failure on the high mileage Sevilles. I'd check those first as mentioned.