I don't have rtl8187 but with my Intel WiFi Link 5100 I get varying signal strengths according to how I measure.
Pre-connect, draknet-center reports signal strength at 10%, but once I connect, it reports 70%. So there is a big discrepancy between the two.
And winxp gets a higher sig strength.
Sorry, not a clue as to addressing the problem, just offering the point about diff signal strength.
btw, I have installed all kernel updates as the are offered & never found any difference. Currently with 18.104.22.168-pclos1.bfs.