Anytone AT-588 UHF tranceiver repeater malfunction

Hello dear fellow ham, madam or sir,

I’d like to put my problem here, looking forward for your kind opinion and (maybe) your suggestions.

My Anytone AT-588 UHF amateur radio tranceiver, transmits normally in full, medium or low power, without problems in the whole amateur uhf band (430-440 Mhz), in simplex mode.

But if I try to transmit, in order to open the nearby repeater (in a distance less than 10 miles), using the 88.5 tone and the -7.600 shift (stored in memory channel 02), my voice is distorted, with blanks (according to the reports of other hams + my monitor from a rearby handheld).

Eventhough, in a simplex test transmission at 431.425 Mhz (the repeater's input frequency), with or without the 88.5 tone engaged, my voice is strong and clear !!!

I tried to reset the rig, from the RESTORE menu twice, but the problem still exists.

My power supply is quite capable in feeding volts and amps, the transmission line is a checked 50 ohms RG 213 cable and the antenna is a factory made Diamond X-50, all at 50 ohms. The rig is about 4 years old, connected inside my shack at all the time (no external use).

Any solution please ??

Thank you kindly for your time !!