Supra stalling

[Kinya]

I have a 1990 Japanese imported Toyota Supra GA-70 (1G-GTEU engine, 2,000cc)twin turbo. After starting, the vehicle will run well some times with no problem. On other occasions it starts well, goes for a while then dies suddenly or idling drops and the engine dies. Initially when this problem started I would observe the idle starting to fall to below 500rpm and I was able to prevent the engine dying by revving it.After stalling am sometimes able to restart the vehicle immediately but some times it just won't start until after quite sometime. The stalling happens mostly when I am in traffic jam or when I slow down. I sometimes smell gasoline when I stop. I have read most of the threads here on stalling and idle problem but I have not succeeded in eliminating the problem. My background is not mechanical but I understand cars and engine operation. I have done diagnosis but it blinks OK without giving any error codes. I took it to a mechanic recently, said it was coolant sensor, changed it but the problem persists. Supras are rare in my country and the mechs here just do not seem to have expertise on them. Please any one with help I will really appreciate!