93 Honda Accord bad transmission with D4 light?

So I have a 93 Honda Accord LX with 288k miles and I am starting to have an issue with the transmission. I know the car and especially tranny are old so I can expect problems, but this is my project car and I want to find what the transmission problem is before putting a new one in. Okay so the problem happens sometime, like 2-3 times a week, when I'm driving around for a while and come to a complete stop. When I try to take off from the stop the transmission would "slip" and the car will rev up to 2000 rpm and will feel like it lost power. If I keep stepping on it like that it will jerk and resume normal operation after like 5 seconds. When this happens at a stoplight I usually put it in D1 and it will take off normally and I will manually shift through all the gears with no problem. The D4 light also flashes constantly while driving when it happens. I want to know if this is a transmission control unit problem, torque converter problem, bad transmission in general or some other shift solenoid or sensor. I have put in Lucas stop slip and it helped for a month, but then started again. I also noticed the tranny fluid is really dirty and dark. Maybe a tranny fluid change with Lucas stop slip might help, what do you think?

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