How did Robocop gradually regain his memories if he had them wiped out?

I don't know for sure, but I think something familiar triggered it (his encounter with the guy at the gas station).

The thing about memories is that once you start remembering something, you tend to remember something else. I wrote about the early years of my life once and by really thinking about it, I ended up remembering all this other stuff I thought I had forgotten. It was amazing. One thing lead to another.

I also read a science thing that said when you remember something you're actually linking to the last time you thought about it, and so it gets filtered over time. This can lead to thinking about other random stuff related to when you last thought about it.

Simplest explanation I can think of was His brain was starting to heal from the injuries it suffered and his memories start to resurface. Even total amnesiacs can have part of their memory resurface over time.

given that his memories were wiped by some totally fictional brain wipe-o-matic machine...
Who the hell knows? what do you want to hear? "there was a mis-calibration in the neuro bleacher catalyst" ?

this is one of the very, very ,very rare times its ok to say "because its in the script!"