Amanda Bynes -- Serious Mental Issues

Amanda Bynes has much bigger problems than smoking pot and driving illegally ... there's evidence she has serious mental issues.

TMZ has spoken with a number of people who have had repeated contact with Bynes over the last few months, and the stories are consistent and disturbing.

Several people from her gym -- Equinox in West Hollywood - tell us, she has been showing increasingly alarming signs of delusional behavior. One person says Amanda came to the gym Thursday looking dazed and began speaking with herself -- making comments and then answering back.

Another person says when Amanda was on the elliptical, she would stop suddenly and start "laughing hysterically for no reason." She did this periodically throughout her workout. This person says he's seen her dozens of times in the last few months, and says it's apparent her condition is "deteriorating."

Several of Amanda's neighbors echo the concern over Amanda's condition. One tells us he observed her on several occasions having long conversations with inanimate objects. Another person says people in her condo complex are aware of her condition and her conduct but haven't called her friends or relatives or authorities because they just don't want to get involved, given her profile.