Be sure you lint filter is clear. Next check the dryer venting. Be sure that the hose is not kinked, or crushed between the dryer and the wall. Very long vent hoses tend to fill up with lint and block the air flow. Keep your vent hose as short and straight as possible.
Lint build up is a fire hazard as well.
Rather than cleaning the old hose replace it with a new fireproof type. Dryer manufacturers recommend not using the old white vinyl style vent, again a possible fire hazard. I recommend using Super Flex venting, which is available at Home Depot and most other hardware stores. When there is a restriction or blockage in the vent it causes the heated air to back up in the dryer. With no where for the heat to go, the temperature in the dryer rises very rapidly.