What about sdcch congestion, sdcch seizure failure rate & sdcch failure rate. Are they high too ?
Have you check counter this 1163/13, 1163/14, 1163/17, 1033 (linked to the radio quality) ?
the best for is to swap the whole TDMA on an another TRX and check the behaviour of the TCH DROP & SDCCH drop.
If all sector of the site are impacted, check trans problem