Oldsmobile Silhouette Electrical / Computer Problem

I have a 98 silhouette with 160K miles on it. The trans was rebuilt last year by a shop that I have used for years and trust, it jost got new tires, outer tie rod ends, alignment, brakes, I replaced the A/C compresoor and accumulator this past summed using A/C delco genuine parts, it has new rear shocks, and is in real nice shape over all. The intake gasket has also been done. The problem I am having now is that the van overheats. When it overheats the guage goes all the way up to the "hot" and the light comes on. If I leave it running and immediately raise the hood, I find that the cooling fans are not running. If I shut the key off for a minute and then restart the van, the cooling fans come on at high speed and cool it down. If I drive it after that, it may work fine, or it may over heat and do the same thing again. Other problems I have noticed that seem to have something to do with this problem is that the engine surges at idle, and the transmission does not shift well. The computer set a code for "fuel system lean", I do not remeber the code number. I thought there was a aproblem with the computer and replaced it with a factory AC/Delco reman, went through the expense of having it flashed, but the problem is 100% the same or maybe a little worse. The thermostat was replaced with a Stant, by the way. I Know jumping on replacing the computer may have been a little premature, but with all the other new parts on the van, and the fact that I need it for work, I would really like this van fixed. The local mechanic shop can't figure it out either. They tell me to take it to the dealer, but my experience in the past of taking any older car to the dealer has not been good. They usually tell me a bunch of stuff is wrong with it like oil pan gasket or valve cover gasket, but can't solve my actual problem, and charge me money that I don't have. PLEASE HELP.