Fan temp switch, works sometimes?? Options

[VavAlephVav]

so I've been nursing a bad headgasket, leaks exhaust gas into the coolant, but doesn't leak into the oil. for the most part I've just had to check and add a little more coolant once a week but it's gotten worse lately.
the system does not leak antifreeze from anywhere, it's only coming back out the overflow bottle. the water pump is only 6 months old, the cap is new(but aftermarket cheapy), T'stat is new and I even tested it in water on the stove with a thermometer.
I have seen for myself that if im sitting there watching it while idling in the driveway the fans cycle on and off like it ought to, but there have been a couple times lately, like today where I check the coolant level before I leave the city and head 30 miles home down the highway. just fine until after 20 miles I come to a traffic slowdown and as soon as I did the temp gauge starts to go up past normal so I pull over and shut it off before it gets all the way up. turn the motor off but turn the key back on and yet the fans are not running when the dash gauge reads above normal (3/4 of the way), I unplug the fan temp switch on the bottom of the radiator and the fans come on and I leave them run to speed up the cooling off time.
so Is it possible that the temp switch works intermittently? or is it the kind of thing that is either broken or not broken? I'd think if its some sort of semi-conductor (variable resistance with temp) then it very well could work intermittently, electronics are like that.
also I have a new cap but its an aftermarket cheapy from advance auto, It says 0.9 on it so I assume that is in Kg/cm^2 , the Toyota manual says it should be 4.3-14.2 psi or 0.3-1.0 Kg/cm^2 . 0.9 is close yet slightly lower than the pressure it should release, furthermore lately after It cools off I check it and the top radiator hose is sucked in(collapsed) under a vacuum. so maybe the vacuum valve doesn't work right either??
I had always thought that maybe the head-exhaust leak was bad enough that it couldn't form a good enough vacuum to suck the coolant back from the overflow, but if that hose is collapsed then It couldn't be that bad.
OR do you think it's possible that because the gauge on the dash comes from the sensor at the top of the motor that it may be hotter because of the exhaust gas in the system and is simply still not reading hot enough for the fan switch to come on at the bottom of the radiator?

so should I stop Effing around and buy a radiator cap from Toyota? but do you think I can get by with an cheap fan temp switch? if oreily wants $25 for it I'm afraid to call Toyota - I had hoped to pick up the head I'm going to rebuild from the salvage yard tomorrow as well - im trying to get it ready and do the head gasket in August.