Hi all, I have a td4 55 plate. When I bought my freelander the temperature gauge never went above quarter and I got no heat from the heater ( short test drive and I never noticed) so I fitted an inline thermostat mod and all seemed to be ok as the needle sat just below half and I had plenty of heat from my heater which was brilliant as it was winter. But as the summer months came along my troubles started and the temperature gauge started shooting up to the red line very quickly( within about 4/5 seconds) and it is very random, I can drive 5 hours up to Scotland and not have a problem but a 20 mile drive and it keeps shooting up, as soon as I put my heater onto full the temperature drops just as quick ( 6/7 seconds). I never lost water until last week and needed to put a litre in to top it up. After a 3 mile drive to work the fans can be going full belt.
As you can probably tell by my description of the problem I am no mechanic but I fail to see how an overheated engine can cool that quick?
Any advice would be greatly appreciated.

Maybe the in line thermostat has failed.