Downshifting problems

[Clydesvx]

I've just developed a downshifting problem. I recently had Timing belts etc. changed which included spark plugs etc. Since then whenever I'm in cruise control and come to a slight grade my tranny slips into 3rd. I know it's 3rd because if I downshift into 3 the RPMs stay the same. When I looked in the manual I noticed most of the sensors connect to the transmission. It also mentions a vacuum line. Could a broken/leaking vacuum line or a bad sensor cause this problem, or is it just time to start looking for a transmission shop?