Flumoxed - CBO/RBO

Hi, I could do with some pearls of wisdom on this one. Win2k server, 8.1.7

I set up a new server for a client a few weeks ago, ran a few tests in our application (USoft), & it was working fine & flying along. Then over the weekend just gone they did the necessary to switch to it as the live server. This basically involved dropping the schema owner, re-creating using the script I used originally, importing a dump from their 'old' server, building the indexes, and running dbms_stats.gather etc.

So as of this morning their users were on the new server, & generally happy with the performance gains over the old one. However, some parts of the application are horrendously slow. It's impossible to know which are good and which are bad at the moment (there's hundreds of screens), but one of the most used screens is taking over 3 minutes to retrieve all the data, whereas it should take well under a second.

Unfortunately they're a couple of hundred miles away, with no remote access, so troubleshooting is not easy. They don't have any Oracle bods on site. Just out of interest, I got them to delete the stats, & suddenly the 3 minute screen was back to it's usual sub-one-second performance. Gathered the stats again, & sure enough it ran like crap.

As the SQL that actually hits the database is generated by USoft, there's no way of using hints. So, question is, why has it suddenly decided to play dead when it was perfectly happy before they loaded the latest dump? More importantly, how can I convince it to start behaving again? The relevant init parameters (sort_area_size/hash_area_size) appear to be reasonable. We've left it using RBO for now, but I've no doubt the phone will be ringing tomorrow to tell me about how brilliant the 'problem' screens are, but now the rest of the system has gone on strike.

I will be spending the evening Googling, but everything you read seems to get blown out the water by you knowledgeable people in here, so I don't know what to trust anymore! I'm not (necessarily) looking for specific things to try, more ballpark areas, if anyone's got any tips.

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Regards,

Preston.