760 GLE misfires

[newbee]

I have had no luck searching this forum, so bear with me if the topic has already been covered and I just havn't found it.

My father-in-law has a Volvo 760 GLE from 1990. 2.7 V6 and automatic gearbox. LH-jetronic engine management system.
When the engine is cold it misfires a few times on his way to work (3 km). No warning lights comes on.
He has also experienced that if he speeds up to fast from a standing start the engine almost cuts out. Not often though.
With a hot engine there is no problems at all.

I'm not sure if that particular model has the "blinker box", which would enable me to check for fault codes.

He was advised to try exchanging the fuel pump relay, and he did, but the car wouldn't start with the new relay, only the presumed faulty one would work. He has also tried cleaning the engine with this stuff you pour in the petrol, but to no avail. He has also exchanged the spark plugs, and spark plug wires.

I havn't experienced the fault myself, but I've asked him if it's a hard of soft misfire, and he describes it as a hard one, as in a faulty ignition, not fuel error.

My question is: Is there any other common faults to look for in this particular model?