$500 for an oil pan gasket

the car runs and drives great. then suddenly hard trembling , especially backing up. i cant figure it out. any suggestions.

Hi, first time poster here... I recently got a 1993 Pontiac Bonneville from my Mother (NON-Supercharged). It ran fine when I got it, but over the course of a month or two has started to run less smooth. The other week I did a cylinder cut out test and found that not one cylinder was mis-firing at idle. I then pulled plugs and checked their condition: all were clean and dry with pretty square electrodes and slightly large gaps. I regapped them and put them back in. During the course of removal, I broke one spark plug wire (I was careful, but it was 'welded' on). I replaced the wires with a new set of AC Delco wires from a local parts supplier. The car ran good and I assumed that it must have been a wire shorting out under a load. Then the car started running less smoothly again. I again checked each cylinder (pulling the wires at the coil packs) and determined that all coils were firing and all cylinders had about the same RPM drop. I checked the fuel pressure and it is about 35-40PSI at idle. I did check the air filter and found it to be in good condition. This car was maintained pretty well. I checked a few intake manifold bolts and found some of them slightly loose. Last week when I replaced the wires, I had checked for codes and found no codes. So... Where to look next? Is there any one item in particular that goes bad on these cars? Any help would be greatly appreciated. -Thanks, Ken