Car Temperature Rising/Engine overheating?

1994 Pontiac Sunbird LE
4 cyl
180 xxx
automatic
Within the last month, had the waterpump/belt, + heater core / thermostat replaced, due to the engine overheating.
The engine no longer overheats, but what happens, is that. Wheni drive for close to 30 mins and above, the car engine temperature will go above the half way mark on the reading, and eventually makes it way to the "red" zone, on the reading. Any idea of waht this could be? I'm a lil worried, to due the fact that I have already spent over $1000, just to fix the overheating issue before. If anyone could help, it would be greatly appreciated!