Juddery Landy TD5 90

I'm new to this game and have a 2000 TD5 90 I bought last year that has been very occasionally juddering for when I put my foot down. Yesterday it got worse. Ironically it was on the way back from a garage who have replaced the rear crossmember (and fiddled with the fuel tank). I don't want to blame anyone at this stage as its not been 100% for a while but symptoms were only very mild. Now the engine management light will occasionally come on and power will drop for a moment. The engine temperature gauge is dropping down (and not going up). I have checked the coolant level and it was a couple of inches below the low level in the coolant expansion tank so I have just topped this up but this doesn't seem to have fixed it. I have noticed that when the engine is fired up for 20 minutes the temp guage is still reading low and occasionally dropping. I think there is an issue with the coolant system. Is there anything obvious I can fiddle with before accepting defeat and taking to a garage??