SDCCH Drop due to radio problems is only detected after several seconds. So the more drops you have, the longer will be your MHT.
So, solve your drops, it’ll solve your long MHT.
What you could do is:
1) lock the TRX that currently carries the SDCCH timeslot with the most SDCCH traffic. For instance if you have 2 SDCCH timeslots, focus on the one that carries the most traffic (you can check SDCCH requests per TRX for instance)
2) if SDCCH drop reduces :
a) either the frequency of the TRX is interfered (low probability)
b) the TRX is faulty (high probability)
3) if SDCCH does not reduce, unlock this TRX and lock the TRX that carries the other SDCCH timeslot. You can always locate a new SDCCH timeslot onto another TRX, in order not to reduce your SDCCH capacity.
repeat step (2)
4) if nothing works, then let me know 🙂