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I would look at the thermostat. It might not be opening fully so you aren't getting proper circulation, which would lead to the car heating up when it is under more load (ie on the highway or going uphill).
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Just to expand on jlomon's thermostat comment. The thermostat is a two way valve. Fully closed it allows quick warm-up by sealing off the radiator system and short-circulates the coolant back through the engine. When the coolant is very hot, it seals off the short-circulating passage to the engine and forces all the coolant through the radiator. When the coolant is neither cold nor very hot, some the coolant short circulates and the rest goes through the radiator depending on intermediate thermostat valve position.
Thermostats can fail open closed or halfway.
OK, that's the theory. But the thermostat is an "el cheapo" throwaway part (about $20) and can be easily changed in 30 minutes. They also seem to fail soon after been involved in any overheating. It looks like your next step?
This is the Coolant flow diagram for the M104 engine. But it clearly shows the function of the thermostat.
