Heart Rate Monitor on Stairs Machine

I use my 935 on the stairs machine in the gym. Whilst working out, the heart rate reading is significantly lower than my actual heart rate, i.e. 90 and 160. However, after I finish the workout in around 30 minutes, the readings would then sudden rise to 150 which reflects my actual heart rate. I have tried adjusting the straps and resetting the watch. Anything else I could do?