Should I downsize a circuit breaker from 40amps to 30 amps?
Rather than replacing the breaker, you should fix the actual problem.  This document (PDF) from the Consumer Product Safety Commission, describes a few safe ways to handle aluminum wiring in a home.
Aluminum wiring itself is not dangerous. The danger with aluminum wiring is with how it's spliced/terminated.  If connections are not done properly, it can lead to overheating and potentially a fire. If terminations are done properly, aluminum wire can be, and is, used without incident.
Replacement
The most obvious recommendation, is to simply replace all the aluminum wiring with copper. However, that is not always the most practical, or cost effective way to handle the situation.
Pigtailing
If the wire cannot be replaced, the CPSC recommends a few alternative methods to terminating/splicing aluminum wire.
COPALUM Connectors
Using a special connector and tool, joining aluminum wiring to a short piece of copper wire (pigtail) can be a safe solution.  This method may require professional assistance, because special tools and knowledge are required.
AlumiConn Connectors
If COPALUM connectors are not available, AlumiConn connectors may also offer a safe connection. AlumiConn connectors are basically insulated terminal block, which allow a copper pigtail to be connected to the aluminum wire. CPSC recommends that this repair also be conducted by a licensed Electrician.