Harvesters becoming idle in Tiberian Sun
There are 3 possible reasons:
When there is no more tiberium left for harvesting, the harvester will shutdown and it won't restart harvesting even after more tiberium is grown.
When your refinery is full of tiberium and you aren't building enough to deplete the tiberium, all of your harvesters will sometimes stop entirely.
Some maps have glitches in the cliffs and slopes that break unit path-finding. Most of those cliffs are fixed on the Cncnet multiplayer maps, but I don't think anyone has ever gone back to fix the missions. (Have you checked out the project called "Tiberian Sun Client"?)
Tips you didn't ask for: If the tiberium is extremely close you should have 2 harvesters per refinery. And you will need 4 harvesters and 2 refineries to be able to build efficiently. (If the tiberium is far away from the refineries then you can have more harvesters per refinery)
Harvesters cost 1400, refineries cost 2000 + a free harvester. Refineries refund 1000 when you sell them, you can do the math.