Why is my iPhone 4s losing cell signal intermittently?

I bought a 4s from someone on a local buy sell site and the phone has this issue....

The cell signal shows full bars for most of the time, but periodically it loses bars (dots) and then loses signal altogether and shows Searching..... after a few minutes 2 to 3, it will find its carrier again and go back to full bars. I am in a great cell area so its not tha,t my other phone carries full bars all the time.

So far I have.

tried the date time manual adjustment.

unseated the battery connection and made sure the antenna is secure. It was.

Looked at the logic board with a magnifying glass and can see nothing out of place. (compared it too a dead board I have)

finally I replaced the antenna speaker component with a new one.

Same problem.

If I turn off the radio with the airplane setting while I have signal and leave it off for a couple of minutes, it will go to Searching and stay that way for as long as 10 minutes until it grabs a bar or two.

Any ideas of what I can try next?

thanks in advance!