I have a 89 YJ with a 400 SBC swap.  I have a 3 core aluminum radiator with an electric fan.  It has a new stock water pump.  There is also an internal auto trans cooler in the radiator.  My jeep will run about 180 while it is ideling, but while driving if i run over about 1800 rpm it starts heating up and won't cool.  Does anybody here know what may be causeing this or have anyideas how to fix this problem.  

Usually a engine runs cooler at a higher rpm. more air flow and more water flow. have you thought about a clogged rad. maybe the electric fan can't pull air threw it. has it always done it or did it just start? and the thermostat could be not opening?? good luck. let us know what you find.  

yep, could be a stuck t-stat...just don't let the 400 get to hot It'll be toast.
the electric fans are a good suplement to a good pully driven fan, I wouldn't run only a electric.  

I've pulled the t-stat out and going to try to run it like that this weekend.  It is a fairly new swap only about a 2 years old so radiator is about 2 years old.  It was a winter swap and runs fine during the winter but in warmer weather it heats up fast.  I do not have enough room to run any other fan and the electric fan is the bigest I can fit on the radiator.  Thanks for the help so far.  

If you did the swap and installed the fan. take a look at how many cfm the fan puts out. this is the air flow. you can get smaller fans that put out more cfm. I agree the electric fan set up is great. but I have seen people buy to small of units and overheat. I had a willys I stuffed a 327 under the hood and it was bad for overheating. all the sheet metal around it wouldn't alow enough air to flow past and cool it down. Good luck.