What do you mean by 'main relay'? The fuel pump replay is the most common cause of this problem, the OVP would not cause the enigne to cut out at speed.

Also check out the fuel pumps, there will be a pair under the car near the right side back seat and possibly another one behind the diff.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)