Time to take him to the repairman ... unless, he is working on regular fabric ... some machines just can't handle the weight.
Sometimes, the machine just gets out of timing and I don't know how to fix it, so I have to take it to a repairman.
The other thing to do is back it off of the full stitch length a bit ... maybe the little lever, if pushed too far goofs something up ...
Are your feed dogs up? (sometimes, the button get flipped and I didn't know it) ...
And, now I am at the end of my abilities to psychoanalyze the machine <g>. But do let us know what you find out, as soon as you find it out.
OH ... one last thing ... when is the last time you cleaned your machine?
Remember that machine I took to the repairman because I had got it out of timing?
Between that time and the next time my machine started doing that, I was working with a gal, and she taught me about cleaning my machine, completely down to removing the plate over the feed dogs. I kept putting it off and then my machine started acting up ... I almost forgot about cleaning out under the foot plate .... at first, I didn't see it, but I happened to tip my machine back a bit, and between the feed dogs ... under the plate ... there was absolutely FELTED fuzz ... so tightly packed that I could have put it on the piano hammers for felt, no less! It took the tip of my seam ripper to go between the feet and lift that stuff out of there ... front and back on both sides. The feed dogs can't work if they can't lift high enough to get above the throat plate (is that what that's called?) ... check it out! Just to be sure ... I am really glad I remembered that one! PHEW!