Sounds like your trap is not filling up.. First thing is make sure 1/4″ pvc tube is in 1″ overflow pipe, make sure your flapper is not leaking, clean out the rim holes, you also could have crack inside bowl, are it could be syphonning because of lack of vent. Let me know.
Art retired plbg, Thanks for the reply, it turned out to be the fill valve. It had failed inside and was not letting enough water thru to fill the bowl. Thanks to your suggestion, I watched it run and saw that no water was going in the overflow tube. Thanks again!!!!