I believe cruise is controlled by vacuum, a small vacuum leak may cause this. Either on the cruise diaghram thing or one of the many lines you have on the engine. You might be able to push in the cruise diaghram thing and plug the nipple coming off to see if it loses air.
I'm not sure if my theory holds true. If you had a small leak, I would think your cruise control would always be going on and off, on and off. How does the car run otherwise? Any lack of power? Maybe its the 140hp and its just not that strong of a motor. Decent size hills you're talking about?
It depends on the hill... Cruise control is designed for highway travel and the steepest grade in the US interstate system in 8% (I-24). If your using it for non-highway travel, then it is going to bog down.
Thanks for all the info. I drive my daughters 2010 cts and my 2001 taurus on cruise control going up hill with no problem, cruise holds perfectly. Hill is "CAJON PASS" IN SO.CA. Just bought this 87 brougham with 51,201 original miles and last registered in 2001, Passed smog check. Been sitting in my daughters neighbors garage all this time and she was not aware the car was even there. Has the locking hub caps with lock tool in glove comp. Dealers tape cassette/gold keys in folder and drivers manuals included and "EVERYTHING WORKS"
I have this same issue, even after redoing my vacuum system. The car simply slows down about 10 mph on hills and then returns to normal. These cars just don't have the pep of new cars, and hills put them under strain whether it's the cruise or our feet. We just have a tendency to gun it and the cruise does not.