ok so my dad just called me and said his car has been doing this awhile but now when its cold its so bad itll spill his coffee. he said i should "check the internet and see if i could figure out what it is" lol. ive been in the car when it does it. its just a bone jarring thunk. its like he has a super low stall torque converter or something. but he said it is getting worse and its only into reverse from park. the rpms arent high or anything. its like 85 at home right now so itll run at about 1k max and drop to about 5-700. just wondering if this is common or not and if his car with only 79k on it is gonna need a tranny soon.

May be the reverse piston isn't moving right. Could be stuck. So when it goes in to reverse the pressure just jumps up instead of going up gradually.