Buffer overflow error in ESP8266_SoftwareSerial library
There's some heavy String usage in that library, would not recommend it for anything you want to be stable, especially with an Uno.

The library seems to create a connection by first clearing the SoftwareSerial buffer and then issuing the right AT command. It waits for 500ms for a response before giving up. I suspect the wait interval is sometimes not long enough for the module to receive, parse and handle the command before responding. So you should try increasing it to 2000 ms at line 773.

The buffer is full message just refers to the library's own buffer for holding received data, which is 300 bytes by default. The buffer's getting filled so quickly (and the SoftwareSerial internal buffer likely overrun too) because you're loading a webpage like Google's which has more than 300 bytes in the response header alone. It's even possible that by the time the next request commences with another AT command, the ESP8266 is still dumping the response from the previous request, and so the command's response is still way down the queue and so doesn't get read before the timeout. 

Instead, try loading a simple webpage like httpbin.org/ip. That is, at lines 952, 954:

char* request =  "GET /ip HTTP/1.1\r\nHost: www.httpbin.org\r\nConnection: close\r\n\r\n";

if (createTCP("www.httpbin.org", 80))