Many times faulty sensors will cause slam shifting like that........and not always showing "engine codes" on the dash, but logging "sub codes" that can only be discovered by using a PCM scanner.........not a reader.
Recently, on my 98 Chevy pickup, I was having similar problems but showing no code at all. A scan of the PCM using such a scanner showed that I had an input speed sensor that wasn't working properly. I changed it out and it shifted perfectly.
A TPS (throttle positioning sensor) will cause slam shifting also (and is very common) but it will usually show a code (check engine).
Taking a ride with someone who has such a scanner (usually $500-$1000) will diagnose the problem correctly, if it is in fact an underlying problem.....
Best of luck
