Heater Control Question

Wife's 97 Jimmy (Same as S10) heat is only luke warm. I've replaced the thermostat with the correct 195 thermostat. Turns out the original was OK. Before I go for a clogged heater core I'm thinking something is screwy with the controls & would appreciate some input. When stepping on the gas as when pulling a hill I can hear vacuum operated vents, etc opening / closing under the dash. There is no vacuum leaks under the hood & motor runs fine. I'm thinkin the control switches might be the cause. Any opinions would be appreciated before I rip it apart.

Thanks,
Gene

I have 1999 S10 extreme and the heater core went bad it was replaced and I must have a pinched vacuum line some were because my temp control dose not work can anyone show me or explain were or which vacuum line it is. ryan