quick changing performance

About a month ago I noticed a slight hesitation in my 89 Supra turbo. Didn't matter if it was at little throttle, or pulling hard. Had it parked in storage for about 3 weeks, and when I got it out again, it ran fine. Parked in our garage, and the next day it ran on about 3 cylinders. Drove over to a heated garage the next day, but decided to go ahead and store at home until summer. To and from the garage, it seemed to run on about 3 cylinders still. Sat after Thanksgiving, I put some flat boards behind the wheels to back up on. Started up, and running fine, hitting on all Six. I'm thinking I have a timing belt that is jumping around a little. Jumping one way at shut off, and sometimes back at start up. Any other ideas?