It could be a timing issue. It has been known to happen. Check it. (your timing) if advanced it will heat at speed.
Generally when an engine jumps time though, it will be very obvious.
Either the engine won't run or it will rough, as the timing will be way off. Retarded not advanced.
It's never a bad idea to change out the timing belt AND TENSIONER on a Zuke. I do mine every 2 years regardless of the miles.
It's in expensive and an easy job.
You may be running lean as well. Check your spark plugs. White = lean, black = fat, brown is normal.
Your clutch fan could be an issue but generally it woud heat at low speed if it was the fan. (Any fan)
Your radiator may need a flush, again every couple of years. Think sediment. Refill it with glycoll ( the green stuff).
Check your hoses . Do this when the engine is running paying special attention to the lower hose.
Does it feel soft ?
Can you se it contract as you rev the engine?
All signs of bad hoses....
Replace your thermostat with a new 180 degree unit.......Again , every couple of years....Boxcar...