Malibu from the depths of hell Help please (transmission/sensor)

spent around $1000 and had a used transmission put in and have been driving it 301 people looked at this and zero comments seems kind of odd not really impressed with it as a whole. stiff ride and something bangs in the back with every bump, that the last 5 people who looked at it can't seem to find?????

oh the crap just keeps landing on my head. I bought a 98 malibu which ran great, new battery, new front strut assemblies and a 2.4 engine with automatic tranny. IN JAN 2012
I knew it needed a brake line and drove it home 16 miles on the e brake without a problem.
long story short I registered the car and decided that all the brake lines and fuel lines were to be replaced. The rear spring was broken so grabbed a pair from a near by bone yard.
Since the 10 day NYS temp sticker was expired (by 5 months) we used a flatbed to get the car to the inspection station this morning. Picked it up and went to a local store about 6 miles, low oil pressure light came on idling in the parking lot, when the gas pedal was pressed it went off. Then got it back home ( 1 mile)and when we went to swap out cars in the driveway lost forward gears ( reverse works fine but all forward gears feel like they slip but move the car)..could it be sensor issues from disconnecting and removing the battery? have to drive it 75 miles for the computer to be ready and pass the emission part of the inspection.
I bought this car for my wife originally, but having second thoughts. any input or thoughts would be appreciated. I have worked on alot of older cars when I owned my shop from 1995 - 2002 kinda stumped with this one.

Thanks
Doug

Went out this morning started it up reverse worked forward worked. When it got up to temp forward would hesitate then engage after a second and go. I'm still thinking sensor