Seen this link here which describes t/c symptoms and mentions shuddering at speeds between 30 to 45mph, which comes and goes. A possible cure for it is an fluid and filter change.
It might be best to pass this, sounds a bit risky. Sofar there's no explanation with regards to the petrol smell in the engine and the fluid appears clean.
Worth keeping an eye out for other GC's, allow taking this risk here, the repair may become costly.