it seems that your threshold of emergency handover due to low rxlev is set too low in the DCS1800 cells. Can you try to increase it by 6dB or 8dB ?
Second thing, you should check your neighbor lists settings : do you allow inter-band measures ? meaning : is a mobile allowed to measure cells that belong to a different frequency band ? (there is actually such a parameter)
finally, you should check the margins that are used in the “cell ranking” (Link factor, HO_margin_lev).. this is maybe vendor’s dependant, but those are usually parameters set per adjacency, and that is used to filter out target cells.
let us know how it gos and good luck 😉