Treadmill slipping?

My Trimline T335H treadmill has been squeeking and now it has started slipping when I run. I have given it a thorough clean and tightened the belt etc. But still slipping. The treadmill is kept in the garage and It first started doing this with the very cold weather before Christmas. Then after using the adjustment allen key, it worked again. Now it has started again. i purchased it second hand from a gym a year ago. Can you advise please.