Good chance that it is your kick-down cable. First try tightening it up a little - maybe 1/8 to 1/4" or so at the adjuster by the throttle spool. This will hold the tranny in geat to higher rpm on upshift and cause to down shift quicker on acceleration. However, if the cable has let go or is about to, this may not help - when they get ready to break, they will usually break one strand at a time and cause the cable to stretch a little knocking it out of adjustment. Adjusting in this case is only temporary as more strands break. With pedal fully depressed (check to make sure no floor mats under pedal!), you should be able to pull out only an additional 2mm of kickdown cable.

Check your fluid - smell burnt at all or look funky? If so flush the tranny, and perhaps drop the pan and clean the valve bodies and such. Use of a synthetic fluid like AMSOIL will help keep the tranny clean and will even help clean up old deposits that could be restricting fluid channels.

Do yo know which tranny you have - AW or ZF? There were some problems with some of the ZF models. IF it turns out yours is a ZF and is toast, you would be better served to replace it with an AW unit rather than repair it.