COrvette 89 hot starting problem

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I have a 89 Corvette with a starting problem. I live in Norway so I have just put it on the road after winter storage ( 5 months ). It starts fine when cold but after it gets up to normal temperatures there is some changes. Service Engine Soon lights up for a while ( and shuts down after some few miles ). When I stop the car end let it idle, it runs up to 1200 - 1900 rpm for a while ( 10 -15 secs ) before it goes down to 600 rpm. If I switch the ignition off an leaves it for a few minutes, it is impossible to start. I have to leave it for about 15 - 20 minutes to cool down before it will start. The starter runs smoothly, it just does not ignite. Same thing happened to me last year after winter storage and I took it to a repair shop. They did not manage to find anythin wrong wit hthe car and it was running smoothly all summer. It souns like some sticky valve or something but I do not know where to start. Appreciate some help.
BR
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