mash temp all over the place

So I have a very simple RIMS system with pump and direct fired kettle MLT wrapped in reflectix. I will eventually upgrade but this is for starters with the money I have now. I regretedly got a cheap thermometer for the thermocoupler and it does not respond very quickly. Anyway, I was going for 152 mash temp. it stayed at 158 for about 5 minutes in the beginning, then dropped to 154, then dropped to 152, so I walked inside for a bit and when I came back 10 minutes later it was at 145, so fired it up a little, and slowly got back up to 152 in a about 5 minutes, where it stayed until about the less 5 or 10 mintes when it started dropping to 148, but then I started with the fly sparge at 168. What I can expect from this?

actually toward the last couple minutes it got all the way down to about 143, but was not there for more than a couple minutes.

Bump. nothing on this?

I'm not familiar with RIMS, but I think I know the basics well enough. When the wort re-enters the mash tun is there a large gap between the end of the pipe and the grain bed? If so depending what the room ( outside) temps are you could be cooling the wort down before it contacts the grain bed causing low mash temps. Did you try using a different thermometer to double check your temp reading? Could be just a bad thermometer. How quickly was the wort leaving the tun compared to how fast it returned? If the wort exited the tun too fast there may have been channeling therefor there wasn't even heating of your mash allowing it to cool down. Did you check your temp in different locations of your tun? That might of helped you prove or disprove the last question. I hope these questions help you figure out what was happening. Also how thick your mash was could have an affect on the whole issue (at least I would think so). One last thought. If you at least came close to your target gravity, I wouldn't be overly concerned about today's brew session. Might help you with the next one though. And what did Pappazan say? Relax and have a homebrew.
Cheers
Kev