Idol and power

Ok so I have the audi a 6 Quattro so when I start it
is say it is almost over heating after few mins its
starta binging saying it is but the rartteaor is not
hot and second is when car is started it runs bit
starts to run roughf like boggs out where almost
dies but then goes back to normal rpms and idols
ok then after few it dose it again then when I go to
drive it it drive fine up to 20mph then starts like
bogging out and jerking and won't go any faster ut
the rpms will keep going up when gave it gas but
won't go past 20 and keeps jerking like it don't
shift or something what could problems be and
the cat smells like its going bad could it make the
car do this hover heat and not drive faster than
20mph