I noticed starting about 1 wk. ago that the temp. gauge on my 1984 244 GL would rise to the middle on initial start-up, in the usual amount of time it takes to reach normal operating temp., stay there for a few seconds and then proceed to slowly rise to between approx. 3/4 and redline over the course of the next few seconds. The needle then stays there for approx. 20-30 sec. and then slowly returns to the 9:00 position, where it stays for the duration of the trip. The upper rad. hose is cool to the touch during the warm-up interval until the temp. needle has returned back to the middle range, at which point the hose progressively becomes hotter- presumably the thermostat is then opening. I have done the following preventive maintenance to the cooling sys. in the past yr.:
1) Coolant flush, replaced with Prestone Ext. Life AF and distilled water, 50:50 ratio, in 1/02 at reputable local Volvo shop.
2) Fan clutch replaced DIY in 5/02, fixed low speed overheating prob..
3) Rad. hoses replaced, thermostat replaced with 191 deg. F/88 deg. C unit (IPD, Wahler brand) in 7/02 at same shop as above. Previous thermostat was Volvo OEM, about 7 yrs. old, but still operating properly.
Cooling sys. then functioned fine from 7/02 until temp. gauge abnormality appeared 1 wk. ago. My impression is that the current thermostat is now starting to fail/stick, but I'm looking for reassurance that I'm not missing a more serious/other cause- i.e. leaking head gasket. The coolant level has not dropped, coolant doesn't smell like exhaust gas, oil isn't "milky," no leaks around water pump. I've never had to change a thermostat after just a few mos. of operation, but this obviously might be the exception. Any suggestions? Any similar experiences? Thanks.

Jeff