probs?

i have a 1991 toyota camry LE V6 2.5L with 147K on it. ive had it for 16 years and it has been running great. for about 3mths now its been doing this thing where if i floor it and it will normally shift at around 5800-6000rpm it will get stuck in like the ping gear. as in when it gets to 5800-6000 it will attempt to shift but it wont and will end up in the redline if i dont let up on the accelerator. could this be a bad sensor or what? cuz the car has shifted at a r5800-6000 perfectly fine before. how much would this cost to fix? in addition the fluid is a light brown/red should i get it changed? could that be the problem?