I had the same problem. It means there is crud stuck in the valve body, causing the TCC solenoids to get stuck on or off. P0741 means TCC is stuck off, P0742 means TCC is stuck on.
P0742 is the worse of the two because it probably means your trans is bucking as it downshifts, trying to deal with the torque demand of staying locked in overdrive. It may stall even. This could damage your trans if left unchecked, and in my case, the problem did not work itself out. It was really stuck.
My transmission mechanic had to replace the valvebody to fix this problem. I think he dropped the unit to do it, but I've heard you can swap (or at least access) the valvebody on some cars without removing the entire transaxle.
If you know someone with a Tech II, there may be a way to energize the TCC solenoid, maybe get it unstuck. I've heard of this being done, but it was not done to my car, and I do not know the procedure for doing it correctly. Good luck, feel free to read through my experience here:http://rivperformance.editboard.com/series-ii-engine-transmission-f4/diary-of-a-dying-transmission-t1594.htm
'98 SC Riviera • 268k miles • 298 HP/370 TQ • 0-60: 5.79s • ET: 13.97 @ 99.28 • 4087 lb • 20.1 avg MPG • Nelson Ledges Lap: 1:30
3.4" pulley • AL104 plugs • 180º t-stat • FWI w/K&N • 1.9:1 rockers • OR pushrods • LS6 valve springs • SLP headers • ZZP fuel rails
KYB GR2 struts • MaxAir shocks • Addco sway bars • UMI bushings • GM STB • Enkei 18" EV5s w/ Dunlop DZ101s • F-body calipers
EBC bluestuff/Hawk HP plus • SS lines • Brembo slotted discs • DHP tuned • Aeroforce • Hidden Hitch
'05 GTO • 49k miles • 0-60: 4.8s • 16.9 avg MPG • Nelson Ledges Lap: 1:26