the overdrive light flashing means that your ECU has just recorded an error code...
now that means 1 of 3 things...
1) ones of ur fluids is low (ie brake fluid)
2) one of ur sensors has reached the end of its lifespan
3) there is something seriously wrong with ur car...
my advise to you is to check your fluids,
and if thats ok, do a diagnostic check of ur car...
heres a good site that tells u how to do that (b4 you do that though, u might want to check if the japanese spec ECU is the same as the US one)
http://mkiv.supras.org.nz/techo.htm#ENGINE_CODES
anyway, if that doesnt show anything, u should try reseting ur ECU...