Buick LeSabre Overheating

So about a week ago my 2002 lesabre began overheating on idle. I
changed out the thermostat, and still running hot. Took it to the mechanic,
and found that my normally-red coolant had become chunky clay coloured
so did a full system flush and replaced the busted water pump and radiator
fan sensors that had gone bad. Car worked fine for a few days, then
started overheating again. I replaced the thermostat again, as the
mechanic had warned me that all the sludge they flushed out was likely to
have damaged it. Still running hot. Got a jug of K-seal and ran it through,
still running hot. It seems fine in idle. but about 5 minutes after I start
driving it. it overheats again. Coolant bubbles slightly in the reservoir so I
thought head gaskett, but I've got no other symptoms of it. No leakage
anywhere that I can find. No white in my oil, no oil in my coolant, no white
smoke or water out my tail-pipe. I'm honestly at a loss. Any suggestions?