I have a 1995 Dodge Neon w/ a 5-speed manual transmission that I drive
to and from work. My check engine light came on and I got three codes
from it:
P0134- O2 Sensor CKT No Activity (bank 1, sensor 1)
P0401- EGR Flow insufficient
U3FFF- No DTC Definition Found

The problem I am having occurs about 30 min into driving the car, from
a cold start. It loses acceleration(in second, third, fourth and fifth
gears), sputters and if I pull over, the RPMs drop below 600 and the
car starts to sputter, like it’s going to just stall and die. If I
turn it off, and back on, the RPMs rev quite high (up to 3500 in some
cases). I am usually at my destination in 30-60 min and after it sits
in the parking lot, it starts up fine, sounds normal, doesn’t have any
problems until about 30 min into my drive home...

I would like to repair/replace any parts myself if possible, but if
not, would like to know which code might be causing my problems so I
can get it in and out of the shop quickly on my days off. Any help
would be greatly appreciated.