it sounds like you PCM (powertrain control module) is screwing up the gear shift pattern. Electronic stuff do get screwed up without any warning (i think PCM are not very costly to replace and one from some salvage car would work just as perfectly as a new one) I guess running a diagnostic (done for free over in auto parts stores) should tell you whether your PCM has registered any error code in there.Another thing is that make sure that your car does not have overdrive unit (i guess hondas dont have it), as failure to run in overdrive mode results in high engine rpm and lower transmission ratio..

But yeah the try changning transmission oil and filter as the first step..and see if the performance changes,,

Good luck bro