Here is the thing with DIGI. And yes, I'm a fulltime 3G user
In Location A, where I stay in an apartment ,few floors up: Signal strength is full. Really good signal strength, and
modem facing window, but speed is sometimes slow.
Complain to DIGI about this, DIGI said they found its coverage issue, and...
nothing can be rectified 
- Basically telling me , just take it and live with it.

Okay fine .
Location B:
Staying in a house. Ground floor, signal is weak, have to put near the front window. Then only can get decent signal .
But if I put the modem on First floor in the living hall of the house (middle) - Signal bar is about 3 bars and sometimes full
And speed is fast. Maybe not much 3G users.
Verdict? Climb too high (referring to Location A) - Signal strength damn good but slow.
Thats why upon next renewal of data plan, downgraded my Internet Cili Padi XL to 28 plan. (But still get free 30 days 2GB , and extra 6GB for renewing early. Thanks DIGI

)
I think it's more on congestion issues than coverage.... many users in the apartment using digi same time?
well if u can find another telco with better services then move to there.... cannot be all 4 big telco sucks same time right
this one more likely got building blocking the signal.... thus go higher get better signal...
speed faster? then less users that area or that particular time? try during peak hours.... see got differences in speed or not?