[ppppenguin]

The '4040 counters are asynchronous. This means that there is a delay between Qn and Qn+1. Over several stages this can add up to quite a large delay. It can easily be measured with a scope.

If you decode a particular count you may see glitches, depending on the actual number you have chosen to decode. This is not the worst problem. If you use this decoded number to reset the counter, each flipflop will reset at a different speed. As soon as the first flipflop resets, the decode will no longer be valid. If the delay through the decoder is less than the worst case difference in reset times of the flipflops, the design can give errors. In practice the reset time difference between flipflops is usually small, and can be shorter than the decode delay. This is especially true with the diode based decoder you have used, where stray capacitance across the resistor will slow down the rising edge of the reset pulse.