2003 Chevy Impala P1811 CODE

I seem to be having two issues with my Impala. Intermittent hard shift from first to second. Transmission shop said transmission shot and needed rebuilt. Only happens maybe once a month. Could it be the PCS?
Also the service engine light never comes on but my ABS/Traction system light does. Was told it was my abs sensor. Security light comes on occasionally and within 2-3 days car will not start, wait 15 min or so and it starts right up.
Now the service traction system light has gone out, the security light has gone out and the car is driving like it was brand new, no more hard shifting and no more service codes. Could it be electrical or the computer?