Coolant System Temp Fluctuation

[LEEVON]

It's been a weird week for my truck. '87 TBI 350, bone stock no modifications. The engine is newer with good compression, runs very strong, no leaks, etc. My only complaint is that it has gotten poor mileage (consistently 12mpg). It has smelled rich at idle, but I haven't investigated yet. I was cruising last week and the temperature approached 240/250 according to the dash gauge. There were no apparent leaks, no crud in the coolant or signs of head gasket problems, and coolant is circulating very well. I replaced the thermostat with a stock 195 and topped it off. Since then the temperature has been fluctuating from cold (160-ish) to hotter (210-ish) regardless of driving patterns which I verify with my infrared thermometer at the water pump outlet and gauge temp sending unit. I know that 210 isn't entirely bad, but it should be holding steady at 195, not climbing or dropping for no reason. I can diagnose/fix the cooling system, not too worried about that.

But here's where I get lost. Immediately after the thermostat replacement the truck started idling very high, at the same time my mileage got much better and the rich smell was gone. I suspected the coolant temp sensor (CTS) or maybe the idle air control valve (IAC) or a vacuum leak or faulty fan clutch was causing the coolant AND idle issues. What I found was that the cruise control unit was holding the throttle open. I have no idea what changed with that or why. I haven't used the cruise or touched anything in that area. I wonder if the old thermostat was the wrong temp, causing the computer to compensate fuel/timing and some wiseguy rigged the cruise to fix idle speed issues.

So to my question; What's the best way to diagnose the cooling system? Pressure test? Ohm check on the CTS? And what's up with the cruise unit being way too tight on the throttle shaft all of a sudden? (probably not going to get an answer on that one, but worth a shot). I want to fix the cooling system, then tackle my idle and gas mileage problem.

Thanks