Recently wouldn't shift into 3or4 gear. Replaced shift solenoid B and did 3/3 fluid change with genuine Honda fluid. Got 3/4 back but it shifts super hard into 3 gear at any rpm . Help?

 

Sorry to say, you may be nearing end of life on your transmission.

 

If I were to get this tranny rebuilt, what sort of upgrades can be done to it to make it a more robust and dependable transmission??

 

As far as I know, virtually all overhaul kits available contain the latest engineering changes. Fact of life, all automatic transmissions will fail at some point as the clutches are designed to wear as time goes on. The best advice is to stay on top of the ATF changes as a means of prolonging the life of the transmission.

 

And maybe add a separate transmission cooler.

 

Not a bad idea; depending on the environment a separate cooler may make somewhat of a difference (here in New England, not so much unless you're commuting in and out of Boston every day), however, if you commute say, in and out of Phoenix every day, yeah, a separate cooler will definitely pay dividends.

 

I was just looking at doing anything you can to help it live longer. We all know HEAT is the main enemy of AT's.

 

As a general statement this is absolutely correct, however, in the case of the Gen 6 4-Speed automatic, it is my understanding things are a little more complicated than that. The two issues I remember reading about back when I bought my 2001 Accord had to do with insufficient ATF flow at idle causing certain parts to overheat (seems to me this was related to 2nd gear), and the quality/content of the composite clutch material. All of the extra cooling in the world wouldn't have helped both of those issue.

 

Well, things have gotten even worse. Driving down the freeway yesterday at 70+mph when it decided to shift down to second gear on its own, rpm instantly over 7k. As I was slowing down and pulling over it was bucking between 2,3,4 gear randomly. Came to a full stop, shut it down, started again and proceeded on (still shifting hard into third gear). I fear its coming to the end of it's life. Tranny shop quoted me $3100....

 

Ouch. Are they rebuilding it too? You might want to look for a good used JDM trans, and see if they would install it. It could save you a few bucks. Just so you know if go looking for a good used one, Honda split the years for the AT to 98 & 99 and 2000 thru 2002. I believe the differences were in the electronics according to the FSM.
Just something to keep in mind.