Heater problems

I have a 1998 sable and when I turn my heat on it works for 3 seconds and then slowly turns to cold air. I took it to a mechanic and was told the problem lies within the dash.
Has any one had this problem?Or any advice?
Thanks

ditto here...i have 98 sable, heater either only gets lukewarm or if it has sit for a while after heating up will blow warm for 20 seconds or so and then cool off.
I'm convinced the heater core is clogged. Try a good strong cooling system flush (i literally told mechanic to 'flush the snot out of it'...helped for a few days then back to old ways.
If you look in your engine compartment where heater tubes enter firewall into dash you'll notice there is also a bypass...so if heater core becomes even lightly clogged the coolant will simply bypass it.
As a last resort I will take it in and get core replaced, but am holding out for other fixes as dash has to be removed, coolant has to be flushed, AC has to be vented etc all to replace the core....AC has worked awesome since day one and I don't want to mess with it!
Will keep you posted...may try pouring CLR or lime away into core tubes.

My 98 sable has the same heater problem. the bypass pipe is hot. the heater core inlet is hot but outlet is cold.
Let me know your solution. Thanks.
Change the heater core will cost $400-500. too much!

Thanks everybody for the input. The heater core was clogged , they said they blew alot of crud out of there.The previous owner had the heater core flushed 5 months ago. How does it get stopped up in the first place? and is there anyway to prevent this from happening again?

I changed the Thermostat, fushed out the heater core, lots of rust came out, no change. I pulled the fuse out for the AC, now it will work for a few minutes without the fan on, once you turn the fan on cold air comes out.

Heater cores have to be flushed separately from the rest of the cooling system. Has anyone tried CLR in their ford? I have read about doing this in MGs with ok results.