98 5.7 gmt400 rpm drop when coasting

[Poopyface]

Started daily driving the sierra and noticing some issues. When coasting down a hill the rpms drop about 300 then it goes back up and it does this every few seconds. The causes the truck to slow down when it happens.

Also noticed rpms surging a little in the opposite manner at idle.

Any ideas?

 

[crisbee1]

Wag, iac.

 

[Poopyface]

IAC was caked so I cleaned it off and will see how it runs tomorrow.

 

[total newb]

What trans?
t/c lock up sil failing?
Brake switch failing?