I smell a burning odor and speed is erratic whats wrong? ?

1. about a week ago for two days in a row, after I had been walking on it for about 15 minutes, something smelled liked it was burning....hasn’t happened since then and I continued to use it.

2. Then I began to notice that the speed is very erratic...starts out at the speed for which I have it programmed but then goes faster, than slower – changing speeds by itself.