If thats the case then its definitely DSLAM congestion or backhaul problem..think about it lor,those time you all lag is most probably ppl p2ping,youtubing etc etc

Check out your SNR and line attn,take the phone up and press any button and hear got static ornot,test with another new modem.

If all ok then definitely backhaul issue

Check your phone filter, your phone line near anything like TV?oven?

If all ok, then call TMnet and say got static when you talk,see whether they bother ornot, sometimes got water in the cable pit will cause this kind of issue especially rainy season