[v8volvo]

Never hurts to start with the basics right?

Engine needs good compression, good supply of fuel, good supply of air, correct base engine adjustment (cam timing, pump timing, valve clearance), and correctly functioning injection pump and injectors to run well.

If it has those things, it WILL run well.

I would make a checklist of steps and go through them systematically if you want to make it right.
- routine maintenance (fluids, filters)
- compression test, and have injectors tested and as necessary cleaned/reset/rebuilt while they're out for that by a qualified shop
- timing check, timing belt replacement if necessary
- valve clearance check, adjustment if necessary
- fuel system examination, check for excessive supply restriction and air intrusion

These things should go a long way towards getting it running normally, regardless of the modifications.