If you can't go wired you should at least take a few steps to help.
If you have an android phone download "Wifi Analyzer" (Apple has something similiar but cannot remember the name).
Check what wifi networks are available, and mainly check the channel they are operating on. By default most use channel 9. Microwave ovens also run on channel 9. Minus the microwave ovens, if there is a spot where someone else's wifi isn't using a channel use that channel. You have to go into your router settings to change it.
Speedtest.net is a great tool, but often results can be misleading. It's checking a large file download/upload.
Pingtest.net will give far better representation of sending data back and forth, versus speedtest where it is just a large file.
When running the speedtest, you should also run it hardwired as well. Probably better off to run it from a computer versus the PS3. We just want to see how good your internet connection is. Same thing with pingtest.net. Also with the speedtest.net you should know what your max download & upload rates are that your ISP is giving you. If they are saying you have a 10mb down/1mb up, and your only getting 5mb down /.5mb up then you've got a problem somewhere. If you do see a big discrepancy with what you think your supposed to be getting, try the speedtest again at off-peak times. Depending on your location to the speedtest/pingtest servers can also effect the dl/up rates. You should get close though.
In the example if you were supposed to get 10mb/1mb and you get a 8.5mb/.8mb I'd call that ok.
These are from my office where I have a 6mb/1mb rate. Now the speedtest server is about 2 miles from my office so I know I'm going to get a good rate from it.
Here's another speedtest to another server about 50 miles away. Still same result. Yeah my Up/Down rates aren't that great, but seems like a decent stable connection.
Packet loss is usually the biggest killer in online gaming. With Ping coming in 2nd.
What else is on your wifi when you are playing? Someone streaming movies over netflix ,download torrents, video chatting. Anything else happening. If you live with a bunch of people that are consistently using the internet for a lot of data, you may want to see about QOS'ing (Quality of Service) your PS3 so the router will let the PS3 have priority over bandwidth.
There could always be other factors contributing to your disconnection.
The star networking model that GT5/PS3 uses is a method that's not the best. It works, but if one point in that star is slow than all others are effected as a result of one person.
Now if you join a fixed host lobby it should be better, but there again depending on the hosts connection.
Any questions you can PM me. Not premium so I can only take so many pms.
Here's something that wasn't mentioned, if your router only supports Wireless N (which a lot of service providers, provide) you're out of luck and will get disconnects. The PS3 doesn't support it (fully anyways). B/G is what you need!
To find out what your modem/router supports, look for a number like "802.11b/g", the 'b' or 'g' is what you're looking for. If it only has an N at the end of it, it's not supported!
Most routers still support the B/G modes out of the gate even if they are labeled as a "Type N" router. You can get into the router settings and force it into one mode only if you need to.