Another Newbie!

[mkIIruckusbass]

Hey guys, yet another newbie. Wanted to say hi to everyone and bring you upto speed on my condition. First of all I wanted to say what a relief it is to find a bunch of guys that seem to be down to earth. Second of all I want to ask my brothers among the MKII Supra domain to forgive me. I have let my first true love to sit on the back burner for such atime that it will be a long time before I can get her to where I want her. Thirdly(is that even word?) I need to figure out a problem that mybe you all can help me with. When I first start my car she idles fairly smooth(for over 275,000 miles as an estimate). The coolant temp gauge doesnt work nor does the speedometer. They both stopped working together on the same night, long story. Once I reach speeds of around 35 mph and around 2000 rpm the check engine light comes on. The cars engine seems to run just as good as did before until I slow down to an idle. Then it idles like turd. A buddy of mine said that it is running in "open loop" after I first start it until it reaches operating temp or a cert speed or rpm. Is there a speed sensor that could be throwing it out of whack or could it be from the coolant sensor?Any help would be great.