93 trooper transmission slipping help

i have a 1993 isuzu trooper ls dohc and for the past couple months the transmission slips going from first to second or it slips going from second to third it only does it when im slightly on the gas but when im about 50% to 75% or more on the accelarator it doesnt do it? what could be a couple different solutions for this? possibly a fliter and fluid change? or what?

fluid/filter change may fix it while your at that check the hydraulic system= valve body and band adjustments and clutch pack clearences clutch pack would most likely be a overhaul though before you start i would recomend making sure you have the proper fluid level when hot to low or even to high can cause slippage also you can try pressure testing or applying the bands/ clutch packs manually with compressed air