debashis

hi,
i own a '96 audi A4 quattro, with a 2.8 liter V6 engine churning
out 172 hp. now here's my problem:
the car is very fast and throttle response immediate when it's cold
outside(during the evenings and on early mornings). it's really a
dream to drive then. but when it's midday or when the temperature
outside shoots up, it's sluggish and a pain to drive..this also happens after the car has been driven continuously for an hour or so. the engine sounds rough and it feels like there's no power in 1st gear at all; so i have to shortshift into second quickly..the throttle
response is really weak and i literally struggle to take it to
50-60mph(that too amid much noise) ...it's slow and raucous and feels underpowered particularly in the first two gears ...could u suggest possible causes
for this? i shall really be obliged if you will e-troubleshoot this
problem..i'm frustrated cos no one yet has come up with a solution for
this...some information that u might need..no check engine light....i thought vacuum leak was the cause, but even after rectifying that the problem persists...it's got new spark plugs and connecting wires and new oxygen sensors as well
please help....thanks in advance