Have a look through my posting. It has a number of different things you could look at, my problem ended up being a failing fuel pump that couldnt hold pressure reliably.
http://forums.toyspeed.org.nz/viewtopic.php?p=744698#744698
It's likely it will boil down to one/some of these being the cause but always check for trouble codes first as that can tell you almost straight away if its a sensor thats at fault.
In no order of preference.....
1) Worn Distributor rotor or cap.
2) Air Flow meter (ST) or MAP Sensor (BT)
3) O2 sensor.
4) Water temp sensor.
5) Faulty ECU - quite common for capacitors to leak.
6) Fuel related issue i.e. clogged filter, fuel pressure (pump), injectors etc
7) Broken or faulty plug leads and/or plugs.
8. Starter signal wire to ECU.
9) Vaccuum leak.
10) Faulty ignitor or coil.
11) Bad earth.
12) Timing is wrong or belt has jumped a tooth.
Behold the mighty SN1FTR.