OK folks, what would you suggest looking at to diagnose the cause of the following fault:

Vehicle drives normally on short journey
About 5 minutes into the return journey, car significantly loses power
Car drives but acceleration is reduced and car simply doesn't "feel right"
Arrive home, idles very lumpy, turn off engine
Attempt to restart engine - tries to fire but won't idle at all

No, it hasn't run out of fuel!

The car in question is a 3.0i turbo Mk.3 Supra. I'd like to check out a few things myself before handing it over to a garage, not least because I don't have home breakdown cover (wish I hadn't driven it home now!).

Side note: Of course it will probably be a write off because I just bought new tyres for it a few days ago. I wrote off my old Jag right after buying tyres (that problem was easier to diagnose - the car was incompatible with the sturdy metal barrier in the middle of the road). Way back I'd just bought tyres for my first mini when the gearbox exploded. I am tyre cursed.

TIA

Rob