I hate for my first post to sound like a sales pitch, but here it is. I've been using the forum to get Bville specific info, and my 2003 has exhibited intermittent hard shifts since I bought it with 80K on it. It now has 172K and I had resigned myself to see which would last longer....tranny or engine. A rebuilt trans installed in my garage is only 1000, and a PCS installed at a shop is 700. I was just waiting until it would fail, then install new trans. On a whim, I was at Autozone and bought a can of seafoam trans tune. Put it in and almost immediately saw the hard shifts reduced to a frequency of less than one occasion in the last week. Prior to the SF, it would predictably hard shift after being driven on the highway and then getting into a stop and go environment. Almost every time. Its gone 2K now with one occurrence. Amazing. It should be mentioned that I've had the trans fluid changed every 30K, no codes in the ECM, and the SF is still in the trans. Next trans service due at 175K.
Thanks to those who mentioned trying it!

Glad to here it'* shifting better. You will need to keep a eye on the trans. A can can't fix a worn part.

I agree. The only thing that this leads me to believe is that there was some sticking or gumming in the PCS that the seafoam cleaned. If it was an electrical signal or failed solenoid, I wouldnt believe a can could fix it. I was skeptical, but $8 was worth the try. At 172K, I didn't have anything to lose. I should also mention that the hard shifts were not subtle. I understand placebos, especially when it comes to additives and anything oils....kinda like when people say their car is noticeably smoother after an oil change. Quieter? Maybe. Smoother? Hardly.

I think your sales pitch is working, I’m gonna try a can very soon to see if it will fix a downshift issue. The trans shifts fine until it gets warmed up and no problems up shifting at all, it’* only when it downshifts from overdrive to 4th. As long as I get on the gas and go it shifts fine, but if I just gently get on the gas then it hesitates or seems to bounce between overdrive and 4th. It’* a bit annoying when going up a hill. So I think a few dollars for a can of this stuff is worth a shot, and since I have no idea when the fluids were changed last, I will try some before the flush.

Not sure how the downshift thing will work out, but so far I'm still giggling.

Hard shifting has come back at the previous intensity. I'm due for a trans service. I've read about installing a shift kit as opposed to replacing the PCS. Any advice? 178K miles...

I am having the same hard shift problems, which a believe are related to the PCS. Reading multiple threads tells me the hard shifts could actually be a result of sloppy shifts. The control system detects the sloppy shifts and turns the pressure up to 100%, which results in the hard shifts. A shift kit, with the resulting crisper shifts, may clear this up. The way I figure it, it is worth investing $100 on a 25% to 50% chance of fixing the issue. Besides, even if the shift kit doesn't clear up the hard shifts, everyone on the Bonneville Forum who has installed a shift kit has loved the results. Even if it doesn't stop the hard shifts and something else has to be repaired, the shift kit is not a wasted expense.

Get a shift kit.

I had the same experience. Installed seafoam, immediate solution for a few days, then problem returned. I then flushed the trans fluid, installed shift kit and went with GM'* latest version trans fluid. 12k miles since and trans is working good.

Were you getting the hard shifts before the kit, and did they go away?

Yes, very hard shifts prior to the shift kit- thought the tranny was going to completely fail. After the kit AND the new trans fluid (Castrol Dexron 6 VI) and new filter the transmission has gotten better over the last 12k miles. It is not like it was when new, but I have full confidence in the tranny to drive anywhere anytime.