2005 Honda Civic with a 4 speed BMXA transmission with 221K miles on it. I owned it since it was almost new and it always had clean fluid. It shifts fine going from 1-2-3 but going from 3-4 it intermittently feels like it was put into neutral for a split second, and then goes into gear. That seems to happen more when the engine is cold. Besides that, it is shifting fine. No trouble codes for the transmission. The fluid is clean and full. Also, what is a normal highway cruising RPM? It runs between 2500-3000 RPM on the highway depending on speed, which seems a bit high but I am not sure. I think I remember it being about 2000 RPM but I am not completely sure. Thanks guys, have a great day.

 

It sounds like the transmission has reached the end of its working life. Automatic transmissions won't run forever due to their complexity. I don't own them for this reason. After working on various cars for 40+ years, I prefer manuals for their simplicity and relative ease of rebuilding. Just remember their are a lot of small pieces in housed in a small package that are susceptible to heat and 1 small part failure can end its life. Good luck.
Regards
Dave

 

Crack in the 4th gear clutch drum/worn o-ring seal is likely. This is very common on overheated Civic transmissions, and units that have high mileage/wear and tear. This problem generally starts with overheated 4th clutches producing a shift flare on apply.

If your engine RPM is too high for a given gear position, the PCM will produce an INCORRECT GEAR RATIO code for the given gear, or a TCC LOCKUP FAULT if the torque converter is slipping during cruising speeds. Since you have no apparent DTC's, you should have minimal slippage.