Make sure yout bios has the bus speed set to 266 rather than 133.
Actually, by default, the processor is set to 100 MHz FSB... in order for full speed to be obtained, it must be set to 166 (166*12.5 = 2075 MHz, speed of the 2600+ model with 333 MHz FSB).
You will have to do it in your motherboard's BIOS (usually, when you turn on the machine, the screen will come up for a key to enter the BIOS setup. Press that key, and you go into the CPU configuration area and select the correct processor (in this case, the 2600+). Two variants of the 2600+ exist, however, in this case, it appears you are using the 333 MHz FSB version (12.5*100 = ~1250 MHz and 12.5*166 = ~2080 MHz)