Auto Transmission Slipping & Speedometer

I have a speedometer problem for about two weeks now. The speedometer sometimes has no response. It drops back to 0 mph when you cruising on the highway or it does not response when you are starting from standstill. When the vehicle is moving, the needle could jump to the current speed, drop back to zero, then jump back to current speed again. This could repeat several times.

The first time the automatic transmission slipped three days ago, I felt like as if I accidentally left the gear in neutral when I was going from first to second gear. I pressed on the gas padel to see the tachometer needle goes up, but I was not going forward. I think it slipped from first to second gear. The check engine light then came on for about 5 seconds before it went away. The speedometer was not responding at the time the transmission slipped.

The second time the transmission slipped. I think it was from first to second gear again. This time the check engine light did not come on, but again the speedometer was not responding at the time the transmission slipped.

Everytime the transmission slipped, the speedometer was in the status of no response. This makes me wonder if there is a speed sensor failure or something which screws up the computer that controls the automatic transmission. However, when the speedometer fails to response sometimes, the transmission is working perfectly okay.

By the way, the automatic transmission fluid was replaced about 5 months ago and the level was still full when I just checked it today.

Question: I am not sure if the transmission problem and the speedometer problem are related or two totally separate problems. According to this page:

"Another cause for inaccurate speed readings is an improper speedometer gear inside the transmission. This can sometimes happen after a replacement transmission has been installed. Most good transmission shops are aware of this and will make sure that the correct speedometer gear is the new transmission."

Could this be a problem with the speedometer gear inside the transmission? But would a faulty speedometer gear causes transmission slipping?

I am hoping that someone with more experience can shed some lights, and my fear is that the transmission needs replacement.

Here is my vehicle info:

1994 Toyota 4Runner V6 4WD 147000 miles Automatic Transmission (never replaced)

Thanks for any comments.

- Superjunkyard