EGR is clean, intake manifold cleaned, N18 has good vacuum, check valve replaced,

the problem, when pressing the gas pedal in park, and in idle or when in drive and idling when gas pedal pressed for a moment there is like a thunk sound. Same when the a/c button is pushed down to turn on the ac. Always happens.

Let beetle idle to warm up for 20 minutes, shut it off and when I started it back up 30 minutes later engine light was on. Took it by to see what code was showing and it said 8121. Is this the thermostat or water pump needing replaced or coolant sensor?
2003 VW Beetle, auto trans, regular gas, 131,000 miles on it. Just bought it last week.

239,000 miles, had new cylinder head, rod brgs, pistons, rings replaced. due to broken exhaust valve. have replaced timing belt every 100k as suggested.

I was only doing 40mph and the belt didn't break. the engine will turn over but the time belt wont turn. How screwed am I? I love my car. i guess its bad. What are some options and prices? Took it to dealership and was told not to bother then was told they were lying to get me to sell it to them. HELP!!!

Just turns over and tries to start but won't.

What would/should my costs be, parts and labor to complete this repair.

Hello, I'm thinking about replacing the injector nozzles in my 03 Beetle TDI. I already chipped it with RC stage 1. My question to the experts would be: What kind of nozzles should I go with (I'm trying to avoid excessive smoke) 184, 205 or 216?