Methylphenidate (Ritalin) is one of the more "original" drugs used to treat "ADD"/"ADHD". As effie indicated, it is quite cocaine-like in structure. It does have a longer half-life than cocaine, but still not a long one. The first dose of the day is likely the one that gets you feeling the best, would I be correct in saying this? When you re-dose, the methylphenidate works, but you are also in the throws of a comedown from your first dose. When you constantly dose throughout the day, there will be sort of a "general" comedown, that the later doses of methylphenidate will not be able to reverse as well. You are left with a series of methylphenidate comedowns, as well as the negative effects of the later doses of methylphenidate such as anxiety, nervousness, uneasiness etc.
If I were you I might suggest taking fewer number of doses, even if you want to keep at around 80 mg, which I still would not really recommend. Stimulants, like methylphenidate, will cause issues at some point. They are neurotoxic and don't encourage the healthiest eating and sleeping habits. They can be of great use when needed, but taking them daily is not something I would do for any length of time. It may be time for you to re-evalute your relationship with the methylphenidate, and perhaps elect another stimulant in its place, or stop taking stimulants on a daily basis altogether. Dexmethylphenidate (Focalin) is quite like methylphenidate, but might be better for you and cause less depressive symptoms. If you really are stuck on staying on stims, maybe an amphetamine would be superior, but that might just dig you deeper into stimulant use/dependence/addiction and what not.