Please check the following: 

The following causes may be considered: 

From inside a building to a housing station

• Attenuation of ADSL signal due to line distance up to the housing station.
  *Electric signal transmitted to metallic cable (analog telephone line) attenuates as the distance gets farther. This is more evident in high bandwidth used by ADSL.
• Attenuation due to noise generated by airport, highway, broadcast tower, high-voltage line tower, etc. 
• Problem with line quality (bridge tap, etc.) 

Inside a building

• Attenuation due to noise generated by television, refrigerator, microwave, audio equipment, etc.
• Too long telephone line which easily gets affected by noise
• Affected by various services using telephone line (in case of Type 1) 

Situation may improve by reviewing the following: 

• If ADSL modem is placed near any appliance which generates noise (computer, monitor, television, refrigerator, radio, etc.), try to keep the modem away from these appliances. 
• If electric/CATV/BC and CS cables are bundled or tied together with ADSL line, untie them and keep them apart.
• Try using a shorter modular cable (telephone line). 
• (If you are using Type 1) If you are using home telephone/automatic gas meter reading device/security system on the same ADSL line, try considering to change the wiring. 
• If there is more than one modular connector within the building, ADSL connection may get unstable if telephone or FAX is connected to each connector. Try using a splitter phone connector to branch the telephone line to connect telephone and FAX.
• If you are using another ISDN line within the same building (this includes when your next door neighbor of the same apartment is using ISDN), ASDN may get affected by ISDN due to wiring. 
• If there is high voltage line/radio tower/broadcast tower near your place, ADSL may get affected and may become unavailable.
• In case of office building, please check again to see if adapter provided by telephone company is placed within Private Branch eXchange (PBX) or Main Distributing Frame (MDF). ADSL is unavailable if PBX is going through any adapter. Please consider using Type 2 or remove the installed device. 

To check the line quality for ADSL, you can refer to the following NTT pages. As an indicator, connection becomes unstable if transmission loss is more than 37 dB.

NTT EAST: Telephone Line Information (Japanese)
NTT WEST: Line Information Disclosure System (Japanese)

If the situation does not improve even though you have tried all the above, please contact Asahi Net English Customer Support.