Since the URC controls worked for a year, the programming is probably OK and you may be experiencing a hardware failure of some sort.

Are both handhelds exhibiting similar symptoms?

While interference cannot be ruled out, the MX850 uses 418MHz while WiFi uses 2.4GHz. It is not likely that a WiFi device will interfere with the MX850.

When you say that it requires 10-15 seconds in order for a command to be executed, is this literally what happens -- press a button once, then wait 15 seconds or are you repeatedly pressing the button for 15 seconds? Do you observe this behavior on all buttons or only on certain buttons?

Are you using the MRF250 or MRF350? Is the MX850 emitting IR emitting any IR directly to the controlled devices?

Make sure that the IR emitters are positioned correctly on the controlled devices. Is it possible that you have a new source of IR interference (new lighting, direct sunlight, etc.)