96 cruise control not kicking in

[Kasa240]

I very recently bought a 1996 240, and within about 2 weeks of driving it, the cruise started acting strange. When I put push the button to set the cruise, then let go off the gas, the car slows down about 5 mph then the cruise kicks in and it fluctuates between the 5 I set it at and the 5 it kicked in at.

Not sure what this really means, I know very little about cars but I am trying to learn alittle. Thought this might be an "easy enough" fix (not that anything on here has been easy enough yet!)

Some info on the car... it runs pretty well, newish radiator, windows dont work, ac is good... had to fix that since I got it though. It only worked on #4, but my boyfriend changed the air compressor resistor thingy and now it works great. Getting two error codes for check engine light: knock sensor and emission system.. fixing knock this weekend.

Thats about it. Don't think any of that has to do with the cruise though.. but then again I know very little as I said before.

Any help is greatly appreciated!

[biggie]

Would start looking at the vaccum lines on all the cruise stuff under the hood in the passenger corner.