Has the garage (really) checked:

All vacuum pipes - all the 3mm ones esp' at hose junctions and all other intake hose junctions for leaks. Intake pipes for blockages. Replace the vac lines with silicone pipe. A vac leak will cause your problem.

MAF fault? Unplug to see if there is a difference.

Coil packs - known 1.8 fault - usually on later models so I've read.

All the above will cause rough idle/dipping although my money will be on a vac leak somewhere..