the crazy air temp is a red-herring, they always seem to read that at the ECU, check out the FAQ on VAGCOM I wrote. Do you know what the ebay chip is? Is it a Poloace one? If so then it should be OK, Alan Laurence who makes these chips knows what he's doing. Otherwise I guess it could be suspect.
You can quickly check for boost/vacuum leaks by disconnecting the boost return valve linkage, then with the engine running briefly close off the BRV (keeping the throttle closed since the linkage is disconnected) the pipework will quickly pressurise and you'll be able to hear any obvious leaks hissing out.