1987 volvo 244 DL idles fine, stutters and backfires at highway cruise speed

Has the outside of the harmonic balancer spun on the rubber damper? That can make the timing way off when the mark shows it to be just right.

'87 is also one of the years with the biodegradable engine wiring harness, I've had to replace several of those, the car will never be dependable with the original harness, the insulation crumbles all over. I rebuilt a few by splicing in all new wires from near the firewall to all the connectors since the cars broke down.

Thank you for the input; I'll check the timing again, perhaps with a better mechanic than I am. Also tonight in a more diligent inspection of the engine, I found the hose between the evap canister and the intake manifold was completely severed. Bought a new length, and will install it shortly. The idling motor control you mention: do I need to remove it to clean it, or can it be cleaned in-situ?

Thank you for the hints! I'll check the wiring for cracks. How would I tell if the harmonic balancer has spun? Is there another reference for comparison? Not too familiar with this one...

Here ...

Yeah definitely replace any cracked hoses, those aren't helping anything.

You might be able to clean the IAC valve in place but you'll save yourself time and headaches by removing it, there's only two bolts to loosen, a couple of hose clamps and an electrical connector.

You can set the engine to TDC by looking at the mark on the cam pulley which you can see if you remove the screw at the top of the timing belt cover and pull the cover away a bit. With it lined up with the mark on the backing plate the mark on the crank pulley should line up with the 0 mark on the lower cover.