Robert,
The main dealer has error codes now of 0340 and 0335. They say they have eliminated the ECU and have now changed the camshaft and crankshaft sensors but fault still remains. Just to recap, if the engine revs above 2500, it cuts off and will not start again for a while but gives no indication why not. It is is possible to drive it at below 2500rpm but again, when the engine is turned off, it will not start again. They are in contact with Hyundai UK technical but are still no further forward or so it seems.
Rick Johnson
This sure is a nasty situation. Indeed those error codes call for crank and cam sensors, but apparently, they are not at fault since they were replaced new. Also, they ruled out ECM (I presume they tested with another ECM???).
I know shops become "lazy" with this sort of problems, because mechanics are paid by pre-established labor/time charts. Situations like this make the mechanics spend many hours on which they are not getting paid, so they tend to set aside the problem and work only when "they have some spare time".
About your car, since error codes are issued and the corresponding sensors were replaced with no results, and also the ECM was replaced, there is (by logic) only one extra situation that may be producing the problem: That is the wiring harnesses.
It wouldn't be strange to discover that somewhere, the a harness loom is knicked, frayed or broken, causing all this problems.
By looking at the service manual, pages 26-27 (FLB.pdf) where the ECM plugs are shown, I notice that both the crank and cam sensor signals both are in the middle plug (E03-2). So there is one common denominator to have a closer look to. It means that the wiring for both sensors go in the same wire harness and loom, which in term, travel the same paths around the car's parts/panels.
Just as reference, this same wire harness and loom (E03-2) carry the signals for the ECT (engine coolant temp sensor), MAF (mass air flow), IATS (Intake Air Temp Sensor), as several injectors, throttle and fuel related sensors. All this is related to engine temp conditions which change according to sensor signals. It explains why after the problem arises, it is hard to start for a while.
By all means, please keep us posted on your findings.