good aftermarket radiator?

This is very simple.

"once yer moven again it'll start comen back down."

This clearly says that you do not have enough airflow at idle. It also says that your radiator is big enough for the job.

What kind of fan do you have? It's less expensive and less work to replace a fan. I would suggest a good clutch fan.

Without a shroud, it's even more important that you have a hood to core support seal. If you don't have one of those, get one.