My baby was like that even in young infancy, except for her, 90 minutes was the longest. This was even if we held her, wore her on my body. Sometimes she would drift off for just 5 minutes at a time. Day and night. We were in a sleep deprived nightmarish haze.
What does your baby do during time with the sitter? My baby started at a sitter for a few hours at 8 months, but it was no better there even though the other babies slept for hours.
I don't have "The" solution for you, but I can suggest that you try a white noise device. One can be purchaced, or a fan or other device that makes a droning noise can be tried.
Over time, we found many problems our baby had. Allergies was major. Not only did she have environmental allergies, but food allergies which led to nutritional deficiencies. A sleep study gave us better understanding of what was wrong as well.
I wish you the best because the lack of sleep can lead to severe problems as it stresses both body and brain. If you cannot get this resolved with normal methods - I'd get allergy tests, sleep studies, nutritional supplements... and whatever it took.
Maybe I am over-reacting, but only because our pediatrician kept reassuring us that this was "normal"... even after I logged all her sleep time. It was NOT normal, and it was NOT "habit".
You are absolutely right to get on this sooner rather than later.