I just got a Tikka Tac A1 and after a couple days of shooting the handguard had worked itself loose.

Here's the issue... When trying to tighten it back up, no matter what method of torquing, bridging the receiver and handguard, etc, the front of the rail always lifts up as I tighten the 2 crossbolts.

Any ideas?

How much movement are we talking about here, and do you have anything mounted on the rail that causes this to be more than an aesthetic problem?  If you can post pics of the issue and the interface in question it would help since I don't have a Tac A1.  I have some experience with the MI rails and I haven't seen the issue.  If the design is similar to the MI, it sounds like either the ID of the rail or the OD of the mounting point are out of spec causing too much clearance between the two parts.  As a result, the rail deforms by the time you get the cross bolts tight enough to hold.  Warranty service may be an option, or they may just tell you everything is in spec.

I'd experiment a little with finding a way to shim between the handguard and mounting point to try to take up some of the gap before you start tightening the bolts.  A single layer of foil tape may just do the trick.  I doubt there's much of a gap, just enough to cause issues.  Bedding the handguard might work too, but I am not a gunsmith and would definitely consult one before trying that.  Screwing that up could get expensive.

Edit:  I think I misunderstood the issue.  For some reason I read "front" and thought "top" of the rail.  My thoughts on root cause above are most likely incorrect.  The advice about shimming or bedding may be useless, or it may not.  I'd say something is definitely out of square, and probably not by much.  Careful shimming or bedding may get you there, but warranty is sounding more likely.  Does Tikka provide torque specs for the rail?

i use one of these,  https://www.coyotelight.com/picatinny-rail-mount/     its works well and is solid.

I attach handguard pushed snugly against the receiver flange attach the rail mount overlapping the receiver pic rails and overlay onto the HG, lightly torque the rail mount bolts for alignment register then torque the HG bolts, followed by a torque all down snugly "goodentight". Other receiver to HG overlapping pick rails would work also, i like the above because of its low height and it does no interfere with my 5-25x56mm scope.

The tac a1 has a weakness in its HG interface to receiver design (IMO) while two bolts are used only one provides useful cantilever (torque) support by riding in a slotted HG attachment flange bolted to the receiver.
Thus they rely on the flange extension mating surface to the HG to provide support, IMO tolerance mismatch can become an issue with some rifles. They should have made the flange collar interface longer so both bolts would ride in slots (like HK and others WTO do it). For me the above rail mount when in stock is a robust cost effective fix I put 1500rounds per season on my 308 tac a1.

notwithstanding its a small issue for such a wonderful rifle

Thanks for the suggestion. I don't think that will clear with my setup (Reptilia 34mm scope mount).

My options are to get a new chassis from MDT or try the Tikka GI barrel nut handguard assembly and throw a centurion rail on there. If I knew the second option would work well I'd do that asap but like all of these custom projects, things don't really turn out the way I hope.

I have the exact tame issue with mine. I've owned mine for almost 1 1/2 years but I just noticed it last month.

I don't like it but I also don't care too much because mine shoots lights out:

What's the torque spec for the crossbolts?  Maybe you're tightening them too much?  Do they need blue Loctite to keep from backing out?

I ended up bridging the rail with a different mount and it worked OK. I don't actually know what the torque spec is but I didn't go over 20in/lb and didn't use loctite. There isn't a spec out there on the net for these bolts. They should easily be able to handle 30ish in/lb based on their size, but I didn't want to push it.

I decided to go with another chassis anyway to better facilitate a clip on optic.