Perform routine maintenence first: Flush system, change thermostat, check rad cap, bleed air (all stated above). You'd be suprised what difference fresh coolant makes. What is the condition of the rad? If the coolant is boogery and it is the original rad it may be partially clogged. Next thing to check on is if the elec fan is rotating in the correct direction, seen them reverse wired several times. Next check your adjustable thermostat for the fan. Don't have one? Get one, it's adjustability will help the car to run more efficiently. Pusher type fans typically do not cool as well as puller types because they are an obstruction before the radiator cooling fins. Next thing to look into are blown headgaskets. Conisdering the upgrades done to the car it would be safe to assume that it wasn't gently treated, just a thought. Good luck!
Edit: Another problem seen often is overheating due to a poorly located/angled intercooler. Try to relocate it or mount it directly onto the front of the rad (go w/ a puller fan). And some quick sheetmetal shrouding works wonders as well.