91 celica GT shudders, anyone know why?

[jagger]

Occasionally my Celica shudders intermittently mostly in the 4th gear. If feels like the engine shuts off and comes back on again.

Recently my car was stolen and recovered. My back seat was missing, but replaced by the insurance company.

I'm curious what's below the back seat, fuel tank? Could the thieves have messed with some something down there that causes the car to shudder? Hoses, regulation device, etc?

The insurance company operator stated that they may fix it, if I can prove that problem was caused by the thieves. Right now I don't even know if the problem is with the engine, transmission or fuel tank.